Do you get more dung exp for killing everything in a floor or should I just kill the once I have too?

The "Level Mod: %" will decrease and even go into the negatives if you leave too many of them alive. Kill everything, but if they're really high and ridiculous, it won't hurt to skip it. The number of monsters you can skip before it decreases your bonus is probably more than you need and it probably increases the larger the floor/higher the difficulty.

What i normally do when i solo, and a lot do while rushing, is leave monsters at dead end rooms alive and just kill the rest. I was doing that and was still getting a positive % at the end of the floor.