Biologically, of course, we are all human. But if "human" is to mean anything moral — anything beyond the purely biological — then some people who have committed particularly heinous acts of evil against other human beings are not to be considered human. Otherwise "human" has no moral being. We should then not retain the word "inhumane." What is the difference between "he is inhumane" and "he is an animal"? Both imply actions that render the person no longer human.
What would Dionne have us call those Nazi physicians — "not nice," "badly flawed," "evil"? Why is rhetorically ostracizing them from the human race "a dangerous path"? He doesn't have an answer because he lives in the left's world of moral-sounding platitudes. Leftism consists almost entirely of moral-sounding platitudes — statements meant to make the person making them feel morally sophisticated. But based on their relative reactions to the sadists of the MS-13 gangs, I trust Donald Trump's moral compass more than E. J. Dionne's.
It is ever dangerous to use dehumanizing rhetoric on people? Of course — when it is directed at people based on their race, religion, ethnicity, nationality or any other immutable physical characteristic. The Nazis did what they did to Jews and others because they dehumanized them based on their religious/ethnic/racial identity. That's why racism is evil. But why is it dangerous to use such rhetoric on people based on their behavior? By equating labeling the cruelest among us "animals" with labeling Jews "animals," Dionne cheapens the fight against real evil.