It's completly understandable that some people want to kill the same enemies over and over, yet if this one aspect of the game can really ruin it for you, you'd better spend a bit more time searching information about the game.

Not all games offer respawns, learn to live with it and don't start whining about how bad the game is cause it doesn't offer respawns.
It's the way the developers wanted it to be, wether you like it or not.