I learned how dangerous dehumanization is when I went on a tour of the Holocaust Museum in DC. (SIde note: I recommend going there. It's not a "happy" place to visit, but it's an important one. Give yourself half a day for the museum and half a day to mentally recover. )

At one point, you can go through or around one of the train cars that transported Jews to the death camps. I walked inside and stopped. The plaque had said how many people were crammed inside, but I couldn't mentally fit that many people in the train car.

Then, I realized my mistake. I was trying to fit PEOPLE. Even though the people I was trying to fit in were imaginary, I was still treating them like people. Once I tried fitting human shaped objects into the train car, I could easily fit that many.

Once you dehumanize a group of people, the door is opened to do anything you want to them because all societal guardrails for the treatment of your fellow humans are removed.