There is something we call forgiveness and total forgetting. Although no one can manage these two. One can go out of the way. We can forgive easily, but cannot forget the pains that have been inflicted on us by someone.
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This same thing is what I can say happened to someone who got his fellow pained after being inflicted with pain by someone else. The best is for one to forgive, but though it may not come for free. When you want someone to forgive the pains you have caused him or her, you just have to accept the blame. At least sorry is not bought in the market.
In some cases, one may put it as a transfer of aggression. Although, since it is in the past, it may be in two forms: either a transfer of aggression or something done accidentally (unknowingly).
Now, if it is a direct one, just as the prompt, such a person may be sparingly held responsible. This may be because he knew what he was doing and decided to pass the anger to the next in line.
In another way, it may be as a result of something done unknowingly, undoubtedly, or maybe the person is passing through one trauma or the other. Or perhaps the person has unresolved pain deep inside of him or her. When he or she is passing through one pain that's yet to be resolved, it may cause unnecessary retaliation. He may see it as nothing compared to what's disturbing him or her, for sure.
Or it may also be that such a person has difficulty in terms of managing his or her emotions. Most people are just too temperamental. Managing their differences and difficulties may be their concern.
These kinds of people need help. They require people who can talk to them just to erase the evidence of not having passion for themselves.
It happened to me some time ago when I was still in my parents' house. A lady whom I saw clearly pushed someone, and that person got hurt. She took the person to the hospital and got her treated. After a few months of treatment, the patient who was hospitalized was better. This means the load of paying hospital bills was lifted from this lady. She began to visit the lady whom she had injured frequently. A few months later, the one who was hospitalized came around to buy something in my street. She was behaving like someone drunk. In another way, they were behaving like someone who had been hurt in the past. So, at the shop he went to, a boy was standing. Maybe he wanted to buy something, no one knows. I was just watching from where I was seated. She pushed the second lady knowingly. I can't say she did not know the boy was there. The fight started. It was a tug of war to accept the pleading from the lady. Although she was talking so rudely.
To be sincere, forgiving such a fellow may not the palatable.