I'd actually argue its worse if their guiding principles guide them to bad behavior, because they have a ready made defense and social insulation around their bad behavior.

That's really at the heart of the evil of the Holocaust. It did not happen because good people were possessed by an evil spirit or tricked by propaganda or whatever, these aren't people acting out of character or at odds with their moral senses. It happened because people allowed themselves to be convinced that an evil was a good, because the propaganda they encountered reinforced their preexisting biases and inclinations.

To the point above, its not the case that fascists lack any set of guiding principles, its that their priorities are perverted from the normal. It's not they don't perceive e.g. murdering members of the designated out-group as evil, its just that their principles either recast those people as less human (making the murder ethically neutral at worse), or that allowing said people to survive will result in far worse outcomes for society at large (making the murder ethically commendable).