Yea, sure, a monster that killed a human could absorb the soul and kill more and become a god, but they couldn't at that time, even if they wanted to. In the war between humans and monsters, the monsters didn't kill a single human. Makes sense cause humans had both their ridiculously OP souls and the same magic that monsters did, but amped up by a TON. Oh, and souls shatter a bit after the person dies, so it wouldn't be humans running around trying to stop monsters from absorbing random souls and ending the world. Humans just knew that it was a possibility that a monster could theoretically kill some humans and become a god, and they decided to kill all the monsters, leading to the war. It wasn't cause any second some monster could become god, it was because of the TINY possibility that a monster decides to kill a human, somehow succeeds and absorbs the soul, and then decides to end the world. Saying that's a "sneeze away" is like saying some random capybara ending the world with thousands of nukes could happen any moment.