It seems that there's a storyline that interests JKR enough that she's done it 3 times--the young man who joins the DEs and at some point finds himself possibly unable to continue. It's the man who gets in over his head. She did it with Snape, with Regulus, with Draco. What's frustrating is honestly it seems like whichever one you like best somehow is supposed to only do the second part--like he had a change of heart without doing wrong first. I was frustrated recently in a conversation about Regulus, though I also hear it about Snape and I know Draco too gets this kind of defense.
They're Death Eaters. Get over it.
In Draco's case, whether or not he's marked or is officially a DE, he's acting as one in book VI. Now, obviously I love his story in Book VI and I do think it's really significant that he doesn't kill Dumbledore. I do think that it's only during the year that he comes to understand the reality of being a DE and for the first time is able to think about what is actually right and wrong, how he feels about hurting other people. Killing for glory turns out to not be a good enough reason, so a threat to his family must be added, though even that doesn't change what he's being asked to be: a murderer. I don't think it's cowardice that keeps him from killing DD--there's no part of the story where I have an easy judgment, like he's just trying to save his own skin or he just doesn't have the guts or he just wants glory.
But I do think that the story begins with Draco as an enthusiastic DE recruit who thinks he's going to kill Dumbledore and that it's going to be great. I don't think when he hears his assignment he thinks, "Oh my, but killing is wrong!" I think he was probably scared, but absolutely thought it was do-able and should be just a case of doing it right, like catching a Snitch. I don't think the moral aspect bothered him at all. After all he was doing something right, he was doing Voldemort's wishes and killing an enemy of the Purebloods etc. So yes, there is a definite element of naiveté there. Draco doesn't really understand what he's agreeing to do. He only finds that out during the year. But he does still agree to murder. It's not a truly informed consent but it is informed.
Most defenses that go too far for Draco, in my experience, are ones that want to put too much emphasis on the threat from Voldemort. I mean, I do think that the threat could have been at least implied from the beginning. Even if Voldemort didn't come right out and say he'd kill Draco and his family if he failed until later under no circumstances was it probably not scary to imagine failing Voldemort. It's just clearly canon that Draco thought killing Dumbledore was something that should have been a good thing that would make him a cool, kind of heroic person, to do. So there is canon that early on Draco wanted to do this task well for himself, not just because he was being threatened.
Now, I do think people often go too far on the other extreme with Draco and basically judge his story as bad right away and never go beyond that. People usually get stuck on "He was BRAGGING on the train!" as if this is some total deal breaker. In a way these two reactions are kind of tied to each other: both of them seem to feel that becoming a "real Death Eater" on some level means you can no longer be redeemed. Draco needs to show some sign that he really always had the same mindset as a good guy all along. I don't think that's the way the story works. To me it's more about someone who genuinely starts down the evil path, which is partially what makes it so scary. And while I don't think Draco's background, upbringing and experiences replace his own decision to join a genocidal terrorist group, it makes him interesting to me and a character I'm willing to root for to make the right choice.
Lately, though, it's the similar defenses of Snape and Regulus that tend to frustrate me. It sometimes seems like okay, we see that JKR is doing variations on a theme with all three young men joining the DEs and eventually having some conflict with them, but all the real ugly bits of the theme are represented by Draco. Snape and Regulus, by contrast, were somehow Death Eaters while never really doing anything we consider bad about Death Eaters. It's like their eventual change of heart must mean they were good guys all along.
In Snape's case the defense often centers on his never really believing the Pureblood ideology. He just wanted a chance to study Potions or something. Do his Dark Arts. Get back at the Marauders. Even in the scene where he calls Lily a Mudblood he's not really being a bigot because he's only calling her that because he's angry at her, he's not angry at her because she's a Muggleborn (which goes for Draco and Hermione too, actually, but the thing is, it's choosing to express the anger through the word Mudblood that makes them bigots). He never killed anybody, of course. And now that we know he's a Half-Blood it's even better--clearly he doesn't really like the Slytherins or the Malfoys or Draco or the Death Eaters, because as a Half-Blood he's immune to Blood Prejudice. Because it would illogical to hate Muggles or Muggleborns when your own hated father was one--especially if you called yourself the Half-Blood Prince. That must be proof he was proud of his heritage (hmm..funny how he's identifying himself with his mother's name there...).
With Regulus I've seen almost the flipside of Snape. As Snape is okay because he didn't really ever believe the Pureblood stuff the way Draco does, in Regulus' case he was better because he did believe the Pureblood stuff where Draco, has personal revenge issues. Draco was making an informed consent to torturing and killing because his father being a DE somehow transfers that understanding to him, even if we see Lucius keeping him out of it. Regulus believed in "Pureblood Rights" but was naturally allergic to murder, torture or Unforgivable curses. He and his family had no idea Voldemort would commit evil acts like that. They just wanted to, in Sirius' words, purify the wizard race and get rid of the Muggleborns and put Purebloods in charge. Err...in other words he was just signing up for the genocide and in no way condoned actual killing.
In a way it makes it seem like that's what makes Draco so damn difficult to get around. He's right there saying hateful things, using slurs, fanboy-ing Voldemort. There's really nothing cool about what he's doing, and we'd rather think of Snape and Regulus as being cool. I just don't think they were. It's not that I think all three of them were alike-- I think one can do really different, equally fascinating characterizations for all three junior DEs, and I think all those characterizations can have sympathetic elements. Certainly Regulus and possibly Snape did have a change of heart and became truly heroic. You just have to also face the fact that your boy is enthusiastically joining the DEs at some point. His change of heart or cold feet is dramatic because it's a real change of heart. The doubts were probably very humiliating for all three, which is why it took some guts for Snape and Regulus to take action. (And will take some guts for Draco if he does that.) But joining the DE is an act with certain meaning. Unless we get canon that shows otherwise, it means some very bad things about the character.
Basically, as far as we know, they all were equally clueless and all equally knew what they were getting into. Oh, and they were all fairly equally proactive. If you like Snape, it's not that Snape was forced into the DEs through peer pressure or didn't know what the tattoo was for while Regulus Petrified Sirius for trying to stop him and walked over him out the door, and Draco woke Voldemort up in the middle of the night and forced the Cabinet plot on him, with Voldemort tacking on the killing Dumbledore part on the way out (the task is to kill DD, and the Cabinet is the means Draco comes up with).
I guess in a way it's a challenge. These characters all have fans-for good reason. So it sort of becomes a challenge of whether you're going to fully accept all the bad things about them, or polish them up. Come to think of it, all the characters in HP offer that challenge. It's just as annoying to have the good characters made the innocent victim in everything as the bad guys. It's just maybe more noticeable when it's a DE that's not so bad.