Well since it's telepathic and knew he was planning to kill him again, and then spared him, I can only assume his promise to change was legit or he would have been killed.
Oh yeah another thing we learned. We saw the interior of the "Cerebus Summoning Chamber". Interesting that he has to pull out a drain in a puddle. I can't figure that out. And when he talked to it and said I'll be outside... was he really speaking to it as a person? That seemed odd to me. Particularly since he knew it wasn't there yet... does it have really good hearing?