The word is a synonym for Megalomania and its meaning, more so the
anti-Megalomania, see Vendetta, with lawful intent of avenging
friends and/or family, with more intensive
power to return.
Gathering from this, we can assume that a Megalovaniac is someone with focused power avenging someone, rather than killing, and gaining power.
He
may have been greatly
powerful, but he didn't count on him having such a
powerful Megalovania, overpowering him entirely.