In the final phase of the Hunter Exam, why does Killua act the way he does? I've just watched episode 20 of Hunter X Hunter, in which it is revealed that:


   During the final phase of the Hunter Exam, Killua kills Bodoro as soon as the fight between Bodoro and Leorio starts.


Why would Killua act in this way?


   He knows that both interfering in other fights and killing other applicants is against the rules.


So I don't understand why he did what he did.
There are a few factors in play, but the most prevalent reason is Illumi's control over Killua. It's possible you underestimate just how much of an influence Illumi exerts—shown both in this episode, and elaborated on in episode 94.

Suffice it to say, Illumi basically raised Killua to be a killer, and thus has had years and years of exposure to Killua to manipulate how he thinks and behaves. Through his aura and manipulative skills (and other methods, revealed in episode 94), he was essentially able to convince Killua that he was not worthy of love or friendship, and that all he was meant to  be was a perfectly honed killing machine.



Essentially, this reversal of psychology drove Killua into an unstable state, unearthing his urge to kill and forcing him to act upon it again. At that point, he had been driven to believe he was just a killer, had failed to protect Gon, and he'd never be a Hunter—why would he care what happened anymore?

(As a side note, it also ensured both that he wouldn't have to fight Leorio, and that Leorio would become a Hunter. So bonus points to Killua for that.)