Shadowdweller said:
Because once the enemy is helpless it ceases to be self-defense and becomes murder.
So, according to that logic, hacking a demon into negative hit points wouldn't be an evil act (in fact, it would probably be a good act), but then watching him bleed to death would be evil, causing an instant alignment change to evil. After all, he's helpless while he's bleeding, and you caused the bleeding. You could easily stop the bleeding.
Which means, in order to remain good, adventurers must always heal their enemies right after they hack them into negative hit points, otherwise they get an alignment change to evil.
A criminal risked his life in the process of committing a crime. There's no doubt that he committed the crime, and there's very good reason to suspect that he's committed at least one other crime, has tried to kill innocents, and will continue to endanger the lives of innocents if left to roam free. Call if vigilante justice if you like, which is neither good nor lawful, but it sure isn't evil when you're sure you have the criminal.
Ogre Mage said:
Good: respects life, even the lives of opponents or evil individuals.
Neutral: respects the lives of others so long as those others do likewise.
Evil: doesn't respect life.
Negative, Good respects
innocent life. They are under no obligation to respect the lives of demons, red dragons, mind flayers, murderers, or mad clerics with an obsession for destroying the world.
This is a game where you kill evil people and take their stuff. There are a thousand and one ways to kill someone in this game, are you telling me that all 1001 are evil if used to kill an evil opponent? It's rediculous.
On the helpless thing: if you disarm an opponent, is it evil to attack him (AoO) when he goes to pick it up? After all, without his weapon, he can't really harm you, so it sure is mean to attack him while he's weaponless, right? Oh, and forget about surprise round attacks, after all, it's not nice to attack someone when they haven't drawn their weapon. And it's really not nice to attack enemies with fireballs when you're invisible and flying and they can't hit you back, eh?
You could
maybe prohibit those things if you had a strict code of honor (i.e. you are having an honorable duel with strict rules), but otherwise, tough luck sucker. Particularly if you initiate hostilities, you have no reason to complain when your opponent defeats you.