Who says you have to kill everything? Sure, maybe if they can freeze or stun you or something then it's annoying, otherwise wouldn't it be pretty easy to run past them considering you are a higher level assuming you have better armour, higher HP or dodge?
Sorry, but it would be stupid game element (no offense to you, just bad idea IMHO). Of course you don't have to kill them, but in that case you are forced to run away from enemies (they will hunt you), then you will pass other enemies etc. - and sooner or later you cant do anything other than to run all the time to avoid unwanted combat. Its EXTREMELY annoying and bad game element (from developer's point of view it's very lame implementation of respawn/enemy AI). Even in action RPGs players should have always enough time to stop and look at surroundings or stop and think about next steps (if enemies are dead). So if you implement respawn to game it basically means that you want players always kill them.
Please, no annoying respawn in RPGs (it can be tolerated only in few special locations like monster nests or deep woods).
