Teacher: Well young one, there are two major reasons.
The first is that some people are victims of an increasingly prevalent societal compulsion to gain alleged superiority by degrading, exploiting, subjugating or marginalizing others. The people with this tendency are usually taught to think this way from an early age and their main fault (aside from their actions) is that they have never had the drive to question what they were taught, expand their worldview and move beyond their ignorance.
Student: Oh..... What's the second reason?
Teacher: Well, some people are just assholes.
The moral of this tale is that uncovering the reasons why a perpetrator does what they do must be secondary to minimizing their damage since, sometimes, there is no real reason and even when there is a cause for the problem, it may be virtually unsolvable.
Apply as needed.