Love, Mercy, Intent and the Killing Curse
"It is my mercy, and not yours, that matters now."
--Harry Potter and the Half-Blood Prince, Chapter 27, "The Lightning-Struck Tower"
This line, delivered from Dumbledore to Draco Malfoy when Draco has the headmaster at his mercy but does not kill him, may very well be at the heart of the denouement of the Harry Potter series. JK Rowling has given herself the freedom, through creating a fantasy world where magic is the preferred weapon and defense, to give love, mercy and intent a role in what occurs when a murder is committed. This is very important, because if mere mechanical weapons were being used by the characters such devices would (assuming that they weren't created by Philip Pullman in the His Dark Materials trilogy) simply behave as mechanical devices do in our world; a gun aimed at Lily Potter would kill her and then the same gun aimed at Harry would kill him. Harry and Voldemort aiming guns at each other would simply cause wounds, fatal or not, rather than the spells locking the wands together in a cage of light. Because of this, the act of killing someone in the Potterverse can be fraught with as much danger and unpredictability for the would-be murderer as for the intended victim.
In the first book we learn that Harry's mother's dying to protect him conferred a protection upon Harry that prevented Quirrell--while possessed by Voldemort--from touching Harry without great danger to himself. In the third book we get to hear exactly what Lily says to Voldemort through Harry re-experiencing that moment when near a dementor; she asks to be killed instead of Harry. And in one of the many interviews JK Rowling gave during 2005, she said that what is important about what Lily did was that she did not need to die; another thing that we "heard" in the third book through Harry's memories of his parents' deaths was Voldemort's voice telling Lily to step aside.
Some readers asserted that it couldn't possibly have been unusual for people killed by Voldemort to have offered to die for someone else; they are probably right. However, the important aspect of what occurred is not just in Lily's willingness to die to protect Harry, but in Voldemort's willingness--although temporary--to spare her. It seems to be that the willingness of someone to die who did not need to die is what conferred the magical protection upon Harry and enabled him to be protected by his mother's "blood" as represented by her sister, Petunia.
In the sixth book, Draco Malfoy is charged with killing Dumbledore, and although he makes two attempts--with the cursed necklace and the poisoned mead--he fails both times and then also fails to kill the headmaster when they are face-to-face and Dumbledore is unarmed and unable to protect himself (after he has immobilized Harry to prevent Harry from being able to protect Dumbledore either). From a psychological standpoint it makes perfect sense that Draco cannot kill Dumbledore in this scenario but was willing to do so indirectly. Humans are quite capable of being given a list of people's descriptions and then asked, as a philosophical exercise, to choose which ones can be left out of a lifeboat leaving a sinking ship in order to give the survivors the best chance of making it. However, if the same people were told that in order to enforce this pecking order they had to bodily throw the "rejected" passengers overboard, to assure that they wouldn't try to overload the lifeboat, most people--thankfully--would balk at doing such a ruthless thing. It is good that humans are wired this way for our general survival; even though Draco was in an untenable position and worried about his own safety and his mother's safety (possibly also his father's) when confronted with the person he had to kill he could not do it. Unlike his Aunt Bellatrix he still has a connection to what makes us all human and so he could not directly take another life.
Throughout the sixth book we receive suggestions and hints that Dumbledore not only is well aware of the attempts being made on his life by Draco but that he most likely expected Narcissa Malfoy to come to Snape to ask for his help and that he ordered Snape to take the Unbreakable Vow so that if anyone killed Dumbledore it would be Snape, not Draco. Dumbledore's intent is very important here, even though, in cases of murder, it is usually the murderer's intent that is most important. In Rowling's magical world Lily's offering herself up to be killed even though she could have stepped aside makes all the difference and Dumbledore ordering Snape to kill him and his intention of protecting Draco from "ripping" his soul through the act of murder also makes all the difference.
In fact, assuming that Snape was following orders when he killed Dumbledore, it is possible that he did not "rip" his soul in the way Dumbledore describes when he is explaining horcruxes to Harry; as such he is not showing less concern for Snape's soul than for Draco's (which also means that Voldemort may have "ripped" his soul through killing James, but not Lily, who was asking that he kill her). Also, because Dumbledore offered himself up when he did not need to die--Snape was reluctant to kill him at the very end and Hagrid's description of their argument seems clearly enough a disgreement over Dumbledore's order to Snape to be the killer--and yet he was killed despite this lack of need could mean that Dumbledore's sacrifice has now conferred precisely the same type of protection upon Draco that Lily's conferred upon Harry, and it is Dumbledore's mercy that is responsible for this.
A possible result of this hypothetical protection may be that Draco will be safest where Dumbledore's blood resides, as with Harry and the Dursleys. Rowling has been very close-to-the-vest about this, but in an interview she did let slip that the headmaster's brother, who has seldom been mentioned in the books, is the barman at the Hog's Head, the pub where Harry went when initially planning the creation of the DA. If Draco is living in disguise at the Hog's Head at the beginning of or any time in the seventh book the reason could be the blood protection conferred upon him by Dumbledore's sacrifice. It is also possible that if Snape--the one who killed Dumbledore--were to try to kill Draco, the curse would rebound upon him in exactly the same way that it rebounded upon Voldemort when he tried to kill the protected infant Harry--except that Snape would be killed, having, presumably, no horcruxes to prevent this. If Snape is truly loyal to Dumbledore he is unlikely to do such a thing, but if the protection extends to enemies in general then Dumbledore's mercy may have been even more effective than suspected in "protecting" Draco--and not just from Draco's soul being "ripped".
So it is possible that we've heard of not one but two instances of people needlessly offering themselves to be killed in order to protect someone else and that willingness to die possibly changing the way that the Killing Curse acts; we know for certain that this occurred when Voldemort tried to kill Harry but we do not yet have enough evidence to know whether Draco received the same sort of "blood protection" through Dumbledore's sacrifice. If this is the case, however, then we must wonder what other sort of love, mercy or intent can alter the way that the Killing Curse behaves, because these things seem to make it vulnerable to change and unpredictable behavior. It is probably quite true, as Barty Crouch, Jr. says (while disguised as Mad-Eye Moody), that there is no counter-curse for Avada Kedavra. However, that does not mean that one cannot prevent it from killing. As we saw with Lily's sacrifice, love and a willingness to die when there is no need can create a kind of "shield" around the person being protected in this way. And we are told in the sixth book, in no uncertain terms, that Harry's "power" that Voldemort does not know, is LOVE.
How can love be a weapon? It can't, technically. Lily was certainly not using it as a weapon; she very likely did not know that her willingness to die would protect Harry but felt that she couldn't just "step aside" and watch her son be killed; she had to go down fighting, even though she was being told that there was no need for her to die. Her love did not permit her to try to save herself, even though she probably expected Harry to be killed whether she lived or died. If this is the result of love for another potential victim motivating someone who was not originally supposed to be killed, what must the result be of loving and/or showing mercy directly to one's would-be killer?
In Rowling's world, largely because of Lily's sacrifice, it seems highly likely that the Killing Curse would not behave predictably under such circumstances. It could even be that the unarmed Dumbledore was offering his mercy to Draco because doing such a thing for your would-be murderer renders the attacker unable to kill the intended victim (apart from whether he is psychologically able to do this); It could also be that the victim showing mercy to the killer would cause the curse to rebound upon the curser, killing them, but it seems unlikely that Dumbledore wanted to risk killing Draco (since he was so interested in protecting his soul) so perhaps it simply makes it impossible for the killer to kill under those circumstances.
In the third book we see Harry show mercy to Peter when Sirius and Remus want to kill him; then we see him attempt to torture Bellatrix just after she kills Sirius, and fail, because Harry's heart isn't in it. Finally, we see him try to torture and otherwise attack Snape after Snape kills Dumbledore; this time it is possible that Harry would succeed were it not for Snape deflecting all of Harry's spells. This sequence is one of the best supports for Snape having been carrying out Dumbledore's orders by killing him instead of Draco; he seems to be protecting Harry from himself as much as Dumbledore is protecting Draco from himself.
What, in the end, would make Harry a true hero is not torturing someone he hates or killing Voldemort; it would be for him to identify with his enemy (something Harry has been learning to do for the last two books) and by doing so come to love that enemy and have the capacity to show mercy. Somehow, during the course of the seventh book, Harry will need to take an emotional journey that will lead to his being able to do this with both Snape and then, that having been the "warm-up", with Voldemort, so that when Harry offers Voldemort his love or mercy at the moment when he is being targeted for death HE WILL NOT DIE and, on top of that, the curse will respond in some unpredictable manner that we haven't seen before. As already mentioned, it seems unlikely that Dumbledore wants Draco to die because he is offering mercy to him; instead Dumbledore might have been doing this with the expectation that, if Draco did try to curse him, the curse would destroy his magical ability, rather than killing him.
If this is what occurs with Voldemort--he curses a Harry offering him love and mercy--he could lose the power he so ruthlessly wields over others. This would be a far better justice than his dying; Dumbledore warned him, in the fifth book, that he had yet to learn that there are some things worse than death. Living without a soul seems to be one of these things; people who have had their souls sucked out by dementors are not really "living" anymore, yet they are, in an eerie half-dead way. For Voldemort, living without his power--the thing he feels defines him--would certainly be one of those worse-than-death fates. It may not even be necessary for all of the horcruxes to be destroyed before this occurs since, ultimately, the "need" for Voldemort's death may be one of the biggest red herrings in the series. Dumbledore himself was said to have "defeated" Grindelwald; his Chocolate Frog Card does not say that he "killed" Grindelwald. If Voldemort loses his magical power he will certainly be defeated in no uncertain terms. No doubt Harry, Ron and Hermione will spend a good deal of time chasing horcruxes while Harry is simultaneously going on the emotional journey that will allow him to show love and mercy to Voldemort. But ultimately Harry's power--love--has nothing to do with destroying a man bit by bit; quite the opposite.
In the end, Harry does not seem like a murderer and is unlikely to become one; he absolutely recoiled from this idea at the end of the fifth book, when he first learned about the prophecy. Snape was evidently trying to prevent Harry's soul from being tainted by torture at the end of the sixth book so it is unlikely that Rowling will allow his soul to be "ripped" by killing anyone, even Voldemort. Instead we know that Harry's ultimate weapon is to be love, and through Lily's and Dumbledore's sacrifices we have received hints of how that power might be wielded to best effect and still leave Harry's soul intact. It seems safe to say that Voldemort has never encountered anyone who, on the verge of being killed by him, offered him love or mercy, so he will not be expecting Harry to do so. Given how full of hate and grief Harry is at the end of the sixth book he could not possibly accomplish this near the beginning of the seventh, but Rowling has about ten months to give him the time to grow and learn and become the man who can do this. We've already seen a little bit of this Harry; now Harry needs to realize that he has this capacity for love and use it with the least-lovable person he knows.
It is his mercy that will matter the most when the cards are down. And he will show mercy and love and be the hero we've all known he can be.