Undertale - Wars of long past

The monsters literally failed to kill any human (if they had killed just one human they could have a chance), it wasn't really a war, it was too one sided, I mean literally one human child can potentially kill the strongest monster, that is why the humans didn't kill all the monsters I guess they felt bad about it and just seal them up.

Didn't the monster's fail to kill anyone, because they're so weak a tiny child can kill them?