The thing was that Asgore made the deceleration in a fit of rage and by the time he regretted it, it had already become a major source of hope among the monsters (we are told and shown various times throughout the game that the idea of getting to the surface is the only thing keeping monster kind going) and Asgore, being the pushover that he is, didn't want to rob them of that. He sought out alternate ways to break the barrier via Alphys which he though went nowhere. In the end yes he did kill six kids because he valued the peace and happiness of his people over them but I think as a king that is a very understandable thing to do (after all real world governments do much much worse things on a daily bases to ensure peace)

The thing about taking one soul and going to the surface is that if he got the next 6 by force would then he would be forced to go through the whole genocide plan because there is no way humans are going to want peace after that and Asgore was trying to avoid that, and about the take one and get 6 more peacefully do you really see any sort of government giving a giant demonic looking monsters six souls in order to become a god even if he just wants to free his people? That's like giving some random dude who appeared out of nowhere a whole bunch of nukes for free because he asked nicely. It wouldn't have worked .

Yes Asgore should have had the guts to tell his people that the war was off and try to find an alternate way and yes Toriel should have gotten off her ass and actually done something instead of being so passive but all the characters in undertale are flawed and the point is that you're suppose to help them overcome that and become better people.