Too Afraid To Ask

Not any one particular reason.

Some folks just want to see others hurt.

Some folks need to place the blame for something on an easy to reach target that can't fight back. A nameless/faceless corporation or government entity isn't satisfying as a target but the person on the mobility cart at the grocery store, there's entire websites/social media pages shitting on them.

Some people are consciously fashy and its just expected that there must be undesirables to be liquidated. Other people are unconsciously fashy and go along with the folks above because its easier to go with the flow than resist.

Some people accept social hierarchies and need groups to put below them on the ranking chart to go, "See! I'm better than [insert group here]!"

Because empathy is an inequally distributed evolutionary advantage.

If I ever had one if those magical wishes, I'd be sure to carefully construct a wish that ensures all of humanity, its descendents, and intelligent creations have a level of empathy that is increases to a level that is at a minimum equal to the modern average of human empathy and with a maximum not to exceed 150% of the pre-existing modern maximum amount.

That cap is important because too much empathy runs the risk of becoming detrimental to mental health. But locking humans at a minimum moderate amount of empathy regardless of any other factors would, in my opinion, DRAMATICALLY improve the state and prospects of our species.

Allow me to attempt an answer.

Of course, there is the simple lack of empathy, the inability to imagine what another person could feel. A complete lack of empathy is very rare, much more common are varying degrees of a loss of empathy which are often situational: "I do not recognise somebody I am talking to on the internet as a full human being." or "This person looks so unlikely what I am used to, they are not fully human."

But far more often immoral behaviour is strongly tied to cognitive dissonance. This psychological concept describes the notion to hold two different, contradictory beliefs at the same time. Such as: "I am morally good." and "I am allowed to hurt others."

The underlying thoughts are often strongly tied to emotions and the way our reasoning works: More often than not, we have an emotion first and find a justification for it lateron. In the cases you are describing that person may feel an emotion/desire to hurt someone. They may know in theory that hurting others is bad, but this is where cognitive dissonance comes into play.

They will attempt to find a rational justification for their behaviour. Confronted with their behaviour, they will use that justification as their defense: "I hurt someone because they got on my nerves!" or "They deserve it, someone had to teach them a lesson." or whatever may seem appropriate in that setting. But this justification is also what they will tell themselves in their heads in order to see themselves as morally good, despite their transgressions.

Of course, the question remains where their desire to hurt others come from. Psychological studies suggest that such behaviour is often triggered by stress of some kind or other (rather than actual malice), which they will then project onto the person they hurt: "I had a bad day and then they made it worse."

Some people regret their behaviour lateron, especially if they are confronted by others; but cognitive dissonance is very strong and prevents many other people from ever recognising their own bad behaviour as such, particularly if their bad behaviour lies in the past. At the same time, the worse the behaviour, the stronger cognitive dissonance becomes; no matter how contradictory that may seem: "I cannot be a bad person, therefore my behaviour has to be justified." And their mind will provide justification(s). It may take a lot of time, effort and critical thinking to undo this state of mind.

I hope my comment answers your question.