Most anime protagonists have a hard line that they won't cross: no killing. While it can be nice to see them display compassion and empathy, it can also be frustrating. Does that villain who just tried to destroy the world really deserve to survive when it's obvious that he's just going to go do the exact same thing the moment the hero's back is turned? Whatever you think of the morality of that situation, it can be trying to watch heroes steadfastly refuse to take lives under any circumstances.
But not every anime protagonist is like that. Some feature anime protagonists who don't spare lives or at least do so rarely. Whether they're hardened gang members like Ash Lynx from Banana Fish, brainwashed children like the kids from Gunslinger Girl, or vigilante fighters who will sacrifice anything for the greater good like Lelouch from Code Geass, these characters know that taking out their foes is sometimes the only choice they have.
Which of these anime features the most merciless warriors? To answer that question, vote them up!