This is worth expanding on. The term doesn’t literally mean “making somebody not a human.”
Most people, even most bigots, would - in a theoretical vacuum - agree with the notion of natural rights: that “being a human” is enough to qualify you for certain rights and dignities.
Dehumanization is the process of eroding that implicit belief. It happens around the edges, and it’s done by degrees.
The citizens of Weimar didn’t live and work and laugh in the shadow of Buchenwald because they thought the prisoners there were literal inhuman monsters. They’d just been given enough of a perceptual turning of the dials to believe that maybe “those people” didn’t rate quite the same intrinsic dignity as “our people.”
That’s what it means to dehumanize an out group. It’s just about twisting dials to say, hey, you don’t have to spend quite so much empathy on them. They’re different. And it all flows out from there, eroding around the edges until your out group goes from different to disliked to vilified.
It’s a process, and demanding that we only act at the end of the process is missing the point. Its the difference between preventative healthcare and cutting off a diabetic’s foot because he drank two gallons of soda a day for ten years.