So if we take free will as our premise, it's clear why God would allow evil committed by human beings. To disallow it would violate the free will that is necessary for moral acts and choices to even be possible.

Umm, what? I thought God was all-powerful. He can create the universe out of nothing but you're quite sure he can't figure out a way to allow both free will and not have significant evil just because you can't figure one out?

I mean, I can even figure out such a way. I can't choose to become invisible or choose to end millions of lives just because I'm having a bad day, but Hitler had enough free will to kill millions of people. Surely God could have given him a bit less. That would certainly still seem to be enough free will to permit moral acts and choices to be possible.

What kind of all-powerful being needs people like you to make sad excuses for his readily apparent bad choices?