Human beings have a seemingly infinite capacity to cause harm to other human beings. History, as one testament to this, is soaked in blood and hatred. The source of the cruelty inflicted by people is the failure to identify with the victim; not seeing them as sharing the same value and having the same ability to feel suffering and joy as you.
This lack of empathy leads to the vilest word ever invented: "subhuman". Such a contradiction (human is not human but still human) is a void that draws the basest cruelty out of people. It can justify such horror and prejudice.
Prejudice has three main stages:
One - Discrimination says, "you have no right to be different". This reveals a deep insecurity about who you are if you cannot tolerate difference; the existence of different people doesn't have to threaten or undermine who you are.
Two - Segregation says, "you have no right to live amongst us if you are different". This can manifest as separate social areas for different racial groups, separate living areas (ghettoisation) and then exploitation of the isolated population (slavery).
Three - Elimination says, "you have no right to live if you are different". Adolf Hitler demonstrated to humanity the ultimate end point of prejudice; the systematic and industrial scale murder of whole populations of people deemed different (holocaust).
Discrimination amounts to punishing the person for their difference, whether skin colour, gender, intellectual or physical limit etc. The assumption of a victim's responsibility for these characteristics is totally misplaced. DNA threw you into the world with these things already determined but they were not chosen by you. Responsibility is a consequence of the ability to choose and this only comes once you are fully in the world (e.g. it is absurd to accuse me of a crime before I was born). Therefore, we are not responsible for those characteristics determined before our birth (e.g. susceptibility to Parkinson's or why we have two eyes). Blaming and punishing people for these things is absurd.
Human beings also have an infinite capacity to love other human beings. This is our contradiction. Love starts with empathy: the ability to imagine yourself as somebody else, feeling what they might feel and valuing your feeling.