[color:"orange"]There are many situations where not killing an opponent is just passing the conflict on to someone else, who (not being the main hero) would be much less likely to be able to handle it. Unless you invent some special places or talents reserved just for the good guys, anything banished can be summoned back, anything teleported away can be teleporter back, or your own allies teleported away. Any animals pacified can be angered and sent back to populated areas, etc.[/color]
If people can be resurrected or reborn (or even animated), killing them isn't as final as it may seem, either. Especially when confronting evil mages, it's as much of a "temporary solution" as the methods mentioned above.
Moral standards, on the other hand, don't need to be those of today. There have been times and cultures in which people deemed it just foolish to grant mercy to your foes or in which bringing back trophies from killed foes even was an important way of gaining prestige within society. I have no problem with a fantasy world that doesn't use the moral standards of today and in which no sane hero would consider leaving an "evil" foe alive. Though admittedly, most RPGs in which the player character kills hundreds of enemies lack an own system of moral standards.