One thing that hasn't been mentioned yet are accidents. Perhaps the villain made a small mistake that had a BIG impact, and now he's working to reverse that mistake no matter the cost.

For example, the campaign I'm planning now has the BBEG accidently kill his father. As his father won't be resurrected, he begins trying to increase his power, eventually discovering necromancy. Now he's seeking to increase the power of Dark in the world so that he will be able to perfectly reanimate his father.