or why don't get even a step further?
give an option for the level boss to "surrender" in some cases. the bosss could assist you at your side , or in exchange of an item give it's live back.
ofcours if you let him/it/her live, you could get the consiquenses later on in the game.
ex. the levelboss made a new army. or got's stronger.
a good example was in the game Cronotrigger (SNES). were you could choose between killing a vampire, or joining him in your party.
