Often when someone is upset, they are releasing feelings which were previously repressed or partly felt in the past. Past events occurred which upset them at the time, based on their mistaken perceptions at the time, and thus led to emotional reactions. Those feelings are coming out and they just have to be acknowledged and felt fully and released.
If you go trying to talk the person around, to talk sense into them, to get them to understand what happened, to explain it or to get them to see things from the perspective or background of the perpetrator, it's not going to help.
These are feelings which were generated in the past and were based on the state of perception at the time. Those feelings need to be released and no amount of intellectual education is going to achieve that or help that. In fact it shuts down the feeling process. Feelings must be FELT, not thought through by well-meaning saviors.
People who have a lot of baggage to feel and clear may take a long time to work through it before they can become ready or willing or able to even get their mind to open up to new perspectives or vantage points. Cognitive shifts will happen on their own as the emotional baggage is cleared out. When the feelings are strong, there is just no way you're going to get "new truth" into the person, it's not where they are at and it's not what they can relate to. It's too much of a leap to jump to the truth and bypass the feelings.
They are in process. You have to give them time to work through it and be patient with their gradual steps. Yes at first we need to feel and release the repressed feelings, and then later after that we might become ready and willing to do some changes to perception, to undo some misperceptions and apply some forgiveness. You can't push it too soon.
No feelings that have arisen as reactions to ego perceptions of victimhood will ever seem to make sense or be true or be justified. They are all false. But they still seem real and are still felt. We have to cathart them and clear out the junk so that we can start to see differently without so much obstacles/blocks in our awareness.
When someone tries to justify someone's actions or explain why they acted that way or play devils advocate or get you to have compassion for where they came from, you just want to tell them to fuck off, because you're not ready for that yet. You need to finish feeling, and even recover from that (which can be draining/tiring), and process the change that the release has brought about, before you can even start to be willing to change your mind.
It's okay to feel. Feel it deeply and fully. Let it out. Release it. Let it go. Recover. Then do some forgiveness.