bannostookaylo:if you kill someone you run the possibility of losing a quest that could have been opened up to you in the future...
The same applies for if you save someone. Their death may have opened up some quests later on but now they're gone.
Sweetest moment in the game? My dog came back from the dead to help me open the Rookridge Demon Door. I fed him a snack and he followed me to see what the treasure was. When I left, he disappeared, having helped his master out one final time.