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

Because they feel the need to make others feel like them, they want others to know how they feel, so they aren’t so alone

Maybe a way that he can desembocate or let out all the pain, but in a really bad way, it can be also a cry for help,

This could be because they feel that it will ease their pain when in reality, it makes it worse. This is not a good method of releasing your pain/anger

This happens due to psychological impact. I have noticed, many people who have had their hearts break by a particular person, would often make it a point to break other people's hearts in the same manner, so that it gives them solace. In the novel, Great Expectations, by Charles Dickens, one of the main characters, was an eccentric old lady named Ms Havisham. Many years ago, her fiance had run away on the day of her wedding, and since then she had been so mentally damaged, that she adopted a girl and raised her to be cold hearted, and made the protagonist of the story, Pip, fall in love with this girl and get mentally damaged the same way as Ms Havisham was. I feel, the main reason behind all this is envy and jealousy, and triggers of the past. Pain can occur to a person, when her or she lacks something, and it is natural for them to feel bitter towards the people who have the very thing that they lack. Hence, they can't tolerate that, and they feel like causing the same kind of pain to them. The mentality here is, "I never had this. Why is he or she having this?"

Maybe they feel like the whole world is already against them and that certain people don't deserve to be happier than they are. It is a bad mentality but it happens sometimes.

They may inflict it on others without realising and/or in desperation to show the person how they feel.

Someone in pain may want to inflict it upon others because they may feel as if everyone around is happier than they are, and they feel insecure about feeling the way they do.

I’d say it’s due to a lack of confidence in themselves. So they feel the need to put other people down to make themselves feel better

people cope in different ways, and most of the time it won't be in a nice way. personally, before i knew i had depression, i would lash out at people and instantly regret it. ill always regret it... maybe they do, too.

Because I care for him/her Because I am concerned about him/her And most importantly I love to think about him/her