3 Second Pseudoessay: On Killing
So, this is a peculiar one for me, because I'll be the first to admit that Kidoumaru is far from being a nice person. In fact, he's a douchebag with very little empathy for those among his fellow human beings who don't wear an assbow and aren't named Sakon, Ukon, Tayuya, or Jiroubou. However, that's not all there is to him, and I feel this bears expanding on.
Kidoumaru is a ninja, more specifically a Sound ninja, and Sound is one of the nastier if not the nastiest Hidden Villages out there. His job entails killing. The attitude to killing of the Sound ninja we see in canon tend to range from over-eager ("Yeah baby I'm gonna splatter your brains allll over the place, party time!") to the utilitarian ("You're in my way, therefore I will take you down, nothing personal"). Kidoumaru seems to firmly be in the former camp; once his teammates are out of the picture, he's not remotely shy about doing Evil Things to the Sasuke retrieval squad. He treats killing them like a game.
Sick, huh?
Let's have a look at that for a minute. Why do that? Kidoumaru isn't a total raving maniac, not really. He's more than capable of being the voice of reason to his comrades, counseling restraint when a jounin who's too powerful for them to take on shows up, reassuring Sasuke (with a hint of a boast) when the prodigal Uchiha expresses doubts. Quite rational. Orochimaru's complaint against him, later on, isn't that he's irrational: it's specifically that he plays around.
So why would an otherwise rational person turn killing people into a game? Well, why does anybody play a game?
Generally, it's something to do when you're bored.
Now, Kidoumaru's INT stats are on par with those of Haruno Sakura, no slouch with the mental mechanics by a long shot, so he probably is the type of person who becomes easily bored. But with killing? It's the ultimate adrenaline rush; you're taking someone else's life. Is he so incredibly jaded by a life of murder that he can't make it interesting unless he mentally arranges it as some kind of twisted RPG? Probably. But he's also only fourteen years old when he dies, and he's done a total of 46 missions prior to that, only 11 of which topped C-rank.
Orochimaru probably wanted his bodyguards acclimatized to murder. And as I will harp on about, if Orochimaru's pattern of collecting loyal minions is anything to go by, he goes after severely damaged kids. Zaku, Kimimaro, even Kabuto...there's no real reason to doubt the pattern. He picks them up, makes them love/fear/live in awe of him, and then trains them to kill and sets them against all comers as his interests suit. Not everybody takes easily to this. Jiroubou is a good example; he has to psyche himself up, and his opponent down, before going in for the kill. A better example is Sasuke: it's revealed in later chapters that despite Orochimaru's encouragement to do just that, when set on by an entire horde of Sound goons he refuses to kill any of them, incapacitating them instead. Orochimaru is obviously rather displeased with this display of altruism.
I more than suspect the Four had to get used to killing on command, with the targets being people who've never done them any specific wrong. Being some of Orochimaru's favoured (as opposed to, say, Dosu's squad), they clearly found ways to do it and justify themselves in it, including tossing conventional morality out the window, with their master's encouragement.
Flimsy, admittedly, but consider as well: the only time he gets really angry, activating his curse seal to Level 2 (which his fellows consider to be the stage he gets bloody creepy at) is when Neji refuses, like a good little pawn, to fall over and lie still. Neji rubs it in Kidoumaru's face that he is fighting a fellow human being with a strong human spirit, not just some "trash game character"...and Kidoumaru clearly cannot handle the reminder, dropping the cool and calculating demeanor from earlier, and kicking in the seal that doubles his aggression and strength, and lessens both his inner and outer vestiges of humanity.
Now, all of this is not to say Kidoumaru is a woobie or something. He's not. Every human being has a choice in how they live their lives (hi there Naruto main thematic elements) and he freely chose to serve an unspeakable bastard and kill his fellow human beings with distinctly sadistic aplomb, hypothetical shitty upbringing or not. But he's not a complete psychopath, and killing in and of itself is not that much fun for him; he's got to manufacture the entertainment in the form of stringing things out and playing his wits against his opponents'.
P.S.-So what does this mean for him in-camp? Well, in the year he's been here, he's had occasion to kill plenty of new and interesting things, especially those presenting an immediate threat, and that's sated his sadistic edge. The other people he's fought, though, he's had no cause to kill, because he's not under orders and (aside from Yoshitsune taking Ros's underthings) they've done him and his no actual harm; the enjoyment's been in matching wits with the opposition, not doing them in, and seeing how they fight back, with the variety of powers and abilities his fellow campers have, has been enough stimulus for him. Which is why he's not itching to kill something for real as hardcore as the twins are sometimes. XD
...h-holy crap is dat sum teal deer. XD