It is hard to understand this if you yourself would never do anything like that.
Often the abuser has been abused himself when he was a child. He identifies with the aggressor and now he is the stronger person and not the victim, and is playing the same role his abuser did when he was young.
Some of these people are psychopaths, people who only look for their own satisfaction and are not able to care for others.
There is no limit to the variations on how humans can think and behave. All through history, people have been doing horrible things to each other. Society has to protect itself. Some of them can be treated with psychotherapy and/or medication, others are untreatable, and for some psychopahts, ordinary psychotherapy makes them more, not less, dangerous.
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Today's date: Wed, 23 Jan 2019 10:17:38 +0100