Mostly hubris. "I did nothing wrong, I have the best lawyers, I will beat the charges."
The fact that he did this instead of executing a planned escape and going to hangout with Ruja Ignatova is the main reason for me to think he believed his own shit.
Leaked his ex-gf's private diaries in an attempt to intimidate her.
Decided to testify at his own trial.
I'm almost certain that he thought he would give a grand speech and all the jurors would instantly take his side, just like he had experienced with VCs, journalists, employees and other sycophants for the past decade.