Board Thread:General Discussion/@comment-5784644-20160220192950/@comment-11139677-20160220194046

Um...this is actually brought up in the game. Toriel was wondering the same thing.

Asgore is too hesitant to kill anyone to be able to do that, and only really compensates for this by making the humans come to him. Even then, it's implied many of the humans didn't make it to Asgore and he had other monsters do the dirty work for him, cementing this further.

That said, things would NOT turn out as well if the monsters were freed before Frisk came along, since they'd be...on much less friendly terms with the humans to say the least.