So Gaster there was the former King during the war

He's not evil, he's not related to the skelebros in any way other than being their king

He wanted to end the war with world peace

Metta, a human (and friend to other Human, Napstablook) met with King Gaster to try negotiating peace terms. Metta was captured and executed. Napstablook was also executed, guilty by association.

Gaster later died, ending the war, Monsters are trapped underground, Sans, a high ranked delegate of King Gaster, is elected King, Papyrus his main commander.

Chara is considered a Prisoner of War, Sans despises their existence. Toriel cherishes Chara, so she keeps her around as a normal person even if they're considered a Prisoner of War. Asriel is the judge but in the genocide run he dies so Chara takes his spot instead.

Any questions, feel free to ask any questions in the comments. (Im still developing the take on the au)