This Wondermark cartoon (in a very different style than usual; see the note to the cartoon) nicely captures the problem of identifying the monstrous. The bull-headed minotaur of mythology might seem like a monster to the knight in the image, but he won't seem like a monster to himself. And the man-headed monster that chases them both probably thinks they are the monsters.

Something to keep in mind whenever you consider somebody "monstrous" in some way: his actions probably seem "human" to him, while yours may seem "monstrous."

At first, I thought this was another angle on "the banality of evil," but perhaps it's not:

In short, the true horror of Eichmann and his like is not that their  actions were blind. On the contrary, it is that they saw clearly what  they did, and believed it to be the right thing to do.

The monsters, that is, do not see themselves as monsters.