He might refer to his post with sixwanderingsouls, and their "unfinished business"
I actually hope he answers that one. He already touched on something similar though, about Asgore recovering from what he thought he had to do.

Asgore was recovering about what he almost did. He and undyn killed six humans or undyn brought them to him I couldn't even imagine the regret once they got outside for the first time.

Yeah, "what he thought he had to do" were the words from that ask. He thought he had to kill them all, so he killed them. He almost killed a seventh human.