It is sometimes necessary to take another's life, in situations of self-defense, because one can be forgiven for doing so in concern for their survival. Those situations can looked over because the intended victim was only acting in accord with their survival instinct.
Self-defense has legal implications, and will draw only sympathy instead of contempt from the surrounding community and media. This law can be exploited, in which case the murder in question is wrong because it was committed with the intent to kill. All in all, what distinguishes what makes killing right or wrong is whether it was done out of intention or out unfortunate consequence.
The matter becomes far more entangled when something like war is thrown into the equation, in which case the question of who's to blame is thrown up in the air for interpretation. And in response to those who said they would kill history's greatest monsters, I disagree. I can empathize with the desire to have revenge on monsters like Hitler, but it perfectly possible that murdering him would have no effect on history whatsoever ( after all, Hitler was also aided by about 4 other people who could have taken his place and used his death to paint him as a martyr and inspire the masses to take up his message with renewed vigor).
And while Aristotle might say that killing once does not make you a killer, but doing so repeatedly does, I think that killing out of a premeditated notion does make you a killer. Trust me, I can guarantee you that most people, no matter what they say, would not want to extinguish something as important as human life.
shit, i pressed yes button on the poll