Think about your favorite villains from the written word. What makes them memorable?
Why else? Because he wanted to bring some ancient evil to earth via Aztec rituals.
Primarily though it was the way he was written. He wasn't just pyscho. He was a psycho who really believed in what he was doing. Not for any altruistic motives, like Magneto, but out of a blind devotion to a religious order, albeit a devout one. He was one of the best kinds of villains, the one who is doing the right thing -- in his own mind anyway.