I would never even come close to the idea of killing an npc without having to, OP: did you play any other RPG before? This can bug a game, and if not you are missing other things like quests/rewards etc!
RPG is about role-playing. Sometimes I'm playing "myself", act in the way I would act if this were real. Sometimes I'm playing knight in shiny armour, never accept any shady deals, never act dishonorably, even if it means I can't do some quests etc. And sometimes, I'm playing homicidal maniac.
RPG is about role-playing. I know some people are playing RPGs without immersing themselves in the game. They just try to do every quest, get every possible experience point. I know people who even save/reload countless times, just to see every possible ending of every quest, to try every solution. But in my opinion, if you're doing it like this - you're doing it wrong. This is good way to play H'n'S, or MMOs, games that are about numbers, leveling, loot, - but not RPGs. Because RPGs are about immersion. If you see everything in one playthrough, replaying is useless. But if you play the game as intended, as the RPG game should be played... Then it is a different game, every time you replay it.
