I don't know if I'm out-thinking myself here, but from the very beginning when all Moai said to the suggestion of leaving was "..." I've wondered if he never had any intention of leaving at all, and onlywent along to convince Mouri not to do it at the last step. There's no proof either way, and it's just as reasonable that he did consider it and ended up not wanting to just like he said.
A pity it turned out like this. I wonder how god will look upon Mouri now.