Why monsters kill people?

Okay, so i just finished the series and what i don’t get about the monsters is why are they killing people?

So if they sacrificed their children for immortality, but are still stuck in the From area, only being awake at night and killing all people for fun - then immortality doesn’t make sense? It’s no fun, there is no benefit and they can’t really enjoy the living forever.

My theory is probably the kimono lady is like a sorcerer that made them immortal but maybe the monsters are actually alive in the real world during daytime while the From monsters are asleep. It makes no sense.

Anyone got a better theory?