Basically he wanted power that he thought he could never have with the good guys, but when he was defeated and usurped by his former master, i think that had a really humbling effect on him. Like addicts who hit rock bottom, he found out he was wrong...
It's very evident that he's a precursor to Kain Highwind. In that sense, he's one of the foundations of FF, not to mention his name was re-used as a surname for a later FF protagonist...