Why it has always be about "inconsistency"? Why can't a character, being pushed over and over again by defeats, snap and start doing all of this?
It's even written on the novels, he went from just wanting Ashenvale for resources to push the Night Elves from Kalimdor to conquer Azeroth from the lesser races. He is a sociopath with extreme power that keeps failing on his goal.
Blizz did stuck to one archtype, the path of a sociopath with daddy issues that was given power without lesson. He parroted Saurfang's words rarely, but at the same time acted the exact opposite (the Alliance sailors massacre near Northrend comes to mind).