So I see two categories of people here. The first is "main character syndrome" - people who see themselves as real and everybody else as not. Self-centered, egocentric people are one flavor of this, and sociopaths are another. Other people aren't real to them in the same way that they see themselves as real.

The other kind is more of a philosophical position. If all that exists is the physical universe - say, matter and the laws of physics - then all that humans can be is matter that obeys the laws of physics. Nothing more. Humans are just a machine made of atoms - a very complicated machine, but still just a machine. They aren't intrinsically different from an AI, or a car, or a toaster. When people accept that position, whether consciously or unconsciously, they dehumanize people, including themselves. This is people in your last paragraph.

And as more and more people slip over into this position, society becomes more and more dehumanizing, because people act the way they think. (Not perfectly, not consistently, but the correlation is greater than zero.)

Upvoted for the clear distinction and elucidation of the categories, but...

I have to disagree that a materialist or physicalist philosophical position is inherently dehumanizing. One can believe that we are just electrochemical mechanical beings made of atoms just like all the other matter in the universe, without having to also devalue ourselves and others. I am human and I like being human! I even like other humans and value them. I just don't think there's anything that special about being a human instead of a bonobo or a dolphin or a supermassive black hole.