The main reason theia mania seemed like a very probable idea was how relatively sane Genesis seemed after meeting the goddess, and the way she went about 'purifying' him of the degradation. She was a Greek goddess, for all they used her Roman name, and the Greek gods seem to like mucking around with people's heads. Normal rules just don't apply when there are gods tromping around, after all. XD
And I'll be honest; I spent most of Crisis Core going, "Why won't you fucking stay DEAD, you insane little cockroach?!" at Genesis. That, and, "Yeah, take THAT, bitch!"-esque trash talk in fights. XD;;;;