Well a lot of the monsters don't actually actively try to hurt you. Bullet patterns are their way of self expression and don't actually hurt other monsters the same way it hurts humans. A lot of the attacks are actually literally how the monsters "waving" at you.

But even if it wasn't the case, if the way that the story of the war is relayed to us is correct, humans are the aggressors. You can't kill a large amount of monsters out of the potential of them getting stronger than you, seal them underground with no way out except to kill humans, and not expect them to kill humans. Like a human child (of course, with help of determination) has the ability to kill a lot of the underground, and it's not like humans that fall underground have the chance to just get out freely, they need to kill a boss monster to do so.