Comparing people to non-humans is dehumanizing, accurately pointing out the evil in someone's actions (or heart depending on how full-throatedly they support those in power committing evil acts) is not.

Calling the Nazi scum parasites or cockroaches would be dehumanizing. Saying these Nazi fucks are evil because they willingly murder people in cold blood to further a political agenda and instill terror in the hearts of the citizenry is not. One literally compares them to an invasive species, the other is a moral judgement on what they choose to do.