In a case of if I were a main character: I'm a pacifist when it comes to killing but that won't determine just how close to death I will beat my opponent to. I won't kill them, I'll let what's left of them (body, mind, and soul) make that decision themselves. If they lose the will to live, it was their choice, but if they live, they better have improved themselves either to be more fun the next time, or to be better people.
I spare my opponents in order to let them live their lives in shame (if particularly nasty). Pride is a hurtful weapon when used correctly. They ever see me again, they will crap themselves silly. In any other case, killing is not my preference. BTW, it stings when you're spared from a killing blow.
Also a personal motto I got from a mod of Unreal (I think the Dark magic mod): Weapons are for the weak. I would do the most of my business like that with my bare hands, thank you very much. Disarm opponent any way possible, then pull off a cherry-tap beatdown bare-fisted, then spare them. Of course, even though I would use good ol' fisticuffs to beatdown my opponent, it wouldn't stop me from using any weapon in my arsenal/in range to disarm them first; figuratively, or literally. Real men fight with their fists, not weapons.
EDIT:
As for the bad guy returning to cause any sort of harm in return, I would make sure to not have any loose ends, or at least leave some as a series of traps in case they are a recurring type. In other words, had I someone I am close to, I would make sure they play the "damsel in distress" role by the book as they're being handled behind my back; only for it to turn out that they're well trained, and just needed the bad guy close enough; upon my return where she'd normally have disappeared, I have the villain in a near-coma again on my doorstep. I would have them (de)briefed on the whole situation just in case. Similar could be said about any possessions too. Traps. Simply enough. The best way to counter a villain is to think like a villain. They fight dirty, I'll respond dirty. Hit me with an ultimatum, I'll hit you with the ultimatum.