Monsters couldn't hurt humans because no actual will to fight/hurt them.
Humans are basically fucked the moment they piss monsters off to the point of fighting.
Considering a monster (Child) only has to kill one person (Also Child) to overpower an entire human village
And after 7 of them are killed by a monster they essentially become god being able to rewrite history and override determination, and the only reason Frisk ever BEAT Asriel was just convincing him NOT to kill him over and over.