Power. Egomania.
Baltar seemed to think he'd be lord and master of his planet. Didn't he claim his colony was to be spared? The Cylons duped him. They manipulated a greedy, power-hungry man into betraying his people with the promise of ultimate power over the last remaining human settlement . . . through the grace of Baltar's favour with the Cylons.
From his lack of remorse, I'd say it's a safe bet to presume he was a sociopath.
At least that's my take.
Today.
