You would understand Seymours motivation if you got the aeon Anima you see his mother was the fayth for Anima and she explains this.
DUN DUN DUNN
Anyway seymours mother left him at a young age to become the fayth and he was sad and blah blah blah so his mother comforted him by becomiong his aeon. He enjoyed this incredible power but soon lusted for more and Anima just wasnt good enough for himso he went on a quest to get power at any cost and that is what made him a villian.
Not the best motivation but yeah.