People would not happily give the souls of their loved ones to a giant seven foot demonic monster regardless of their intentions. that's like Satan appearing out of nowhere one day and saying "hey give me 7 souls so I can become a god and free my demonic people I SWEAR WE ARE NICE!" It wouldn't happen

His motivation wasn't because he was afraid of being seen as wrong so much as dashing the hopes of his people (hell he even tells you this when he says he just wanted his people to have hope and I don't know why so many people ignore this over Toriel who is biased as hell and hasn't even been there for decades and has no idea what is even going on or what Asgores real motivations were) that they would finally be safe and able to live without having to worry about humans screwing them over again. Humans had already wiped out hundreds of monsters in the first war that humans started, sealed them underground and then killed their beloved prince over a misunderstanding. The monsters have zero reason to believe that everything wouldn't just happen again if they were freed and tried to live peacefully with them (Gerson says as much during the genocide route when he says that they had all agreed to stay trapped at first) At the time there is no other realistic option other then they stay trapped forever or they wipe out the humans so they can live in peace.

Remember that a large thing that Frisk does during TP is show the monster race as a whole that humans aren't all jerks and that they could live alongside them.(mostly through the Mettaton fight) In several endings where you kill too many people the monsters rebel and throw Toriel off the throne because all you do is prove that their pre-conceived notions are correct.

The characters are more complex then Asgore was bad and Toriel was good and I wish people would actually see that instead of just assuming that someone is terrible and worthless because one character who was mad at them said so.