I've debated this on and off throughout the past half decade or so. I think it's something ALL people wonder. For me the reason I wonder WHY suffering happens is to find a solution so it DOESN'T continue. Most people on the other hand take the WHY and then pass judgments and condemnation. Because the WHY we are debating is why a PERSON causes anther to suffer.
First off, are ALL violent acts the same? You take away the severity of the pain, it then becomes about the nature of the crime itself. And NO they aren't all the same.
Categories of Crimes of Suffering:
1. Societal Crimes: Racism, Sexism... Wrongful, Judgement Based Crimes Based on IGNORANCE
2. Sexual Crimes -Usually about Dominance of POWER
3. Pure Violence like Murder -To end a life quickly -ESCAPE/END CONFLICT
4. Slow or Torturous Crimes -Deep psychological reasons like EMPTINESS (You don't take an act out slowly unless you're enjoying the process or feel it needs to be done)
5. Crimes of GAIN -Done for Money or some other MEANS TO AN END
6. Beating/Ongoing Abuse -Maintain POWER
So the ultimate reasons range from IGNORANCE, GAINING POWER, ESCAPING ISSUES, GAIN ITSELF, and EMPTINESS. Gaining power seems to be the most popular one. Especially in our society. And you can't help but wonder if it's because on some level we push people to that. It wouldn't be happening on this wide a scale if certain people in our world -specifically men -weren't being pushed towards AGGRESSION and POWER. You're not a real "man" if you are SUBMISSIVE or SUB-Anything. You have to be ALPHA.
I don't think it's instinctual like through genetics or "breading". And either way humans aren't animals and have rationality.
There is also a distinction -which our society fails to recognize sometimes -between PSYCHOLOGICAL factors and MENTAL factors.
Psychological factors are emotions, SOUL, past issues, society... EXTERNAL influences turned INTERNAL in a negative way.
Mental factors are more geared towards how your physical brain operates and processes information. Like motor skills. How you process things logically and analytically.
There's a difference between Henry David Thoreau and Einstein. One writes from emotions, past experiences, and spiritual insights -the other from cerebral thoughts and science.
The problem is society sees Brain and Psychology in BOTH and assumes they are one in the same or there is overlap where none exists. Society itself knows too little about the subject in general to really pass valued judgements on any of it.
And the other issue is sometimes there is Overlap. Sometimes a mental disorder is paired with psychological past trauma.
There are many underlying contributory factors to be understood. Again I'm not a psychiatrist and I actually don't like them much in general anyway. They tend to analyze people like insects under microscopes and think too much with their head and forget the compassion of their souls.
Ultimately it comes down to making the person doing the harming itself to UNDERSTAND why they are doing it. Then from there you can allow them to figure out what IMPACT it had on their lives and the lives of those they hurt.
This is Retribution. Not "eye for an eye" in the sense of taking something from someone who has taken -but allowing them to discover EMPATHY for themselves. THEN they will not harm in that way again.