I also didn't expect that, but then I realized that's the work of a teenage boy with a catholic education!

Teenage boys love badass, edgy stuff. And what's badass and edgy in Catholicism? Demons! As for the art style, it is the style that was popular at the time.

In a sense, it is not so different from today's kids drawing scenes inspired from their favorite comic. Of course, the painting here shows incredible talent, he is Michelangelo after all, but that doesn't make him less of a kid.