Obviously all speculation, but I'll bite.
From what I understand, the F/O remained silent. He didn't call ATC, he didn't deviate course, he didn't respond to the CA, he didn't start a suicide chant. To myself, this accident reeked of a hypoxic/depresurazation event like the Helios 737 crash. And perhaps, that's what he intended. The 320 hit hard enough to send the FDR chip flying out of the box, and they had one hell of a time getting the CVR read out from what I've read. The bastard almost got away with it. Hell, even with the CVR when it was released that one of the pilots was locked out, I'll be honest and say I still didn't think it was intentional, I figured it was an unfortunate chain of events. Maybe he wanted to kill himself but didn't want it to look like he killed himself.
Of course, short of a note or confession, we'll never know for sure. We can't get inside his head. He's dead. As is everyone else who had the misfortune of being on board that day.