On Nightmare! difficulty (or on Angel of Darkness), monsters respawn continuously unless you destroy their corpses.
However, one can make use of this to fill kill rate for some challenge,
that is, suppose that you have 300/500 (60%) kill rate so far and only 1 enemy is remaining in that floor.
One can think to kill it and wait until it respawns, and kill it again and again, ...
so that to extend his kill rate slightly larger again and again.
If you do so 1000 times, then the kill rate becomes 1300/1500 (86%).
If you do so 10000 times (I wonder if how crazy guy will do this, but, anyway)
then the kill rate becomes 10300/10500 (98%).
After making it sufficiently large enough, then you can get large enough kill rate even after skipping all of later levels.
So, I think that there must be a limit of respawning mosters (like 20~30 times),
or a limit of counting kills(of course with generating counts, simultaneously) of same monster.