How did he manage to be both amusing and completely terrifying

his amusing-ness contributed to the terrifying part. He was so all powerful he could afford to be a little goofy. And he acted quite differently between the two occasions. When he was goofy his vocal tone would go up and down and he would generally talk more. When he was being terrifying he was mostly monotone and would only dole out a sentence or two at a time.

Characters who are out just for themselves can ham it up, but can also be scary.