It was my first thought to make the key appear after all the monsters died. But I wanted to leave some randomness, so you could pass the floor by opening the key and the door at the same time:)
Yeah, that makes sense.
I've noticed that I generally want to clear the entire dungeon in hopes of finding food or weapons, but not always: sometimes I know there's only misery left, and then it can be really helpful to have an unlocked door to flee through. You can sometimes get that unlocked door with your key/door system, whereas a blood-quota system would *never* give you an escape route until you've fought the monsters. I think the keys are working as intended.