Perhaps. You mention that he finds out he is not a descendant of the Cetra, but rather, the descendant of an evil creature. When he found this out, he was already unhinged. He had also been chilling for several years in the Lifestream, doing very little besides absorbing knowledge. So, after the realisation of his true ancestry as a planetary destroyer and armed with the knowledge that he had received, becoming a God is quite a plausible way for him to act.

Or he could have just been really irritated that Cloud beat him and thought this would be a great way to get revenge.