"The only good Gods are the dead ones. All the good ones die in the end, so why not hurry it along and kill the bad ones too." God Killer said. "And that wasn't always my name. I believed in something once."
Right now, there are only 2 ways to get God Killer on your side.
You can't do the first (I mean you could, but you would need exterior information)
and the second would be to convince him that your way would kill the most Gods. God Killer views gods as a Math equation. If you summon a god then kill it, that is like you did nothing. If you summon a god and kill 2, he sees that as a good thing.