Asgore sighed, "Yes, I started the war out of anger and my wife became disgusted with me." "You also admit you killed some children." "Yes." Asgore bowed his head, "I am so ashamed." Sure you are, thought the officer. That's why you almost killed Frisk. The officer wasn't sure how much that was planned, so Frisk would advocate for the monsters.
After all interviews were through, many monsters were placed in jail for their crimes. Levels vary. Though some were able to plea to stay underground.
Sans and Asgore got the worst of it, being placed in maximum security prisons. Though, Sans slightly worse as the jury did say Asgore waited until the children came to fight. Their eyes continued, even if the fight was forced by them being unwelcome.
The humans opted to separate the guilty monsters and put them in separate prisons. Sans was forced to wear a copper anti magic bracelet. He was surprised humans still had those things, he thought they forgot they even had magic. Clearly, he was mistaken.
"At least Papyrus only got community service." He mumbled, being escorted to the jail. Being a skeleton does not make getting searched any less embarrassing or any more comfortable. Especially as thorough as the guard was.