It's clear that the only way out of this is to kill Lee Minhoon. He hates humans, but he's added a human into his family, which means that he can't kill them anymore. Plus, he doesn't have the power to kill his son anymore. So, he can just capacitate Lee and take him away from the family. That's it, says Lee. He's not going to be able to kill the family any more. He can just kill them all if he pleases. So that's why they're working with Lee. They don't know if Lee's going to change his mind in the future, but they'll have to wait and see. They're not sure if it'll work for now, but it's a good idea for the future.