He has already started his defense. I read where he told police that he did not have his bi-polar medication that night because he was kidnapped. Now, if he commited this crime, he obviously wasn't kidnapped, but he could make noises about the bi-polar condition being a primary reason for his actions. Taylor's mom has thought it was Ben Fawley all along.
He's going to try the insane defense. I don't think it's going to work, tho. I think the tons of stuff they took from his apartment has some incriminating details that he's not going to be able to explain away.