Why would someone who is already in pain wish to inflict it upon others?

Some people sometimes feel as if no one around them understands them. Therefore, they spread their 'sadness' with those around them to make themselves feel slightly good.

Someone who is in deep pain and has friends wants to look strong when they're weak and want to look tough so they put the pain upon others to look superior and better than them or they also could want to take out the pain on someone else or to make others suffer the pain they suffered and went through to feel better about them self and feel normal and the same as others

The person choose to do this because he/she is frustrated at their lack of control in their own situation and to help relieve that stress and to make themselves feel better they respond that way.

This is because the hurt person wants others to feel the way that they are feeling. This is a result of a person not wanting to feel like they are alone.

People often do this as a defense mechanism. If someone is experiencing pain, he would feel inferior or helpless. To overcome such feelings, he would start taking advantage of other people to feel more superior. It's a person's way of compensation.

It's a coping mechanism. Their minds go to thinking maybe "venting" it like that onto others, without realizing it'll hurt them too.

because they are full of pain and want others to feel how they feel

Maybe that person feels as though no one around them understands what they are going through and makes threats to inflict pain on others to make them feel what they feel.

Sometimes the best way people feel they can help others understand their pain is if they feel pain too. A person in pain may feel that others do not experience pain as they do, therefore they will inflict pain to make sure people understand. Another reason some people may inflict pain is because since they're not happy, they don't feel others should be happy as well. Not always do people want to inflict pain, they just don't know how to express their own pain in a healthy and productive manner. Feeling pain is human and a part of growing.

2 causes, but only one cause after you see the details. 1. lack of understanding about karma, that's a first. having a bad intention, to hurt others, you plant new bad seeds for the future. But the person doesn't know or cares about this, if the person "falls" deliberately into these thread of bad activity. 2. irresponsibility. Believing that others are the cause, or were the cause, for the pain that the Person feels right now. It's misleading, and quite hard to see this fact. Because mind mostly, because how mind works. Always creating confirmations about what you believe or want to believe. But, clearly, such person believes others are the cause, environment is the cause, person gives power to others this way (which seems as a short term good deal, gives more peace of mind, but long term is a disaster :D) Both points after all they are exactly the same. You are the cause for everything around You. Mind does it for your, time develops the seeds, environment grows them, but You plant everything.