I have a story in which the main character is a serial killer, killing people who have committed crimes but never been caught or were not convicted as guilty. They do this because over the years they developed a twisted sense of justice. This only worsens when their best friend is murdered, and the characters co-worker seems to be hiding information about the case. Due to this, the main character distrusts their co-worker and believes them to be their friends killer. Also, the co-worker suspects something is off about the main character but they dismiss it because they seem like a nice guy.
I know mental breakdowns happen because of a set of circumstances, such as a trigger, and one of them is most certainly the death of their friend. However, I want their breakdown to be in a courtroom where other characters are finding out who actually murdered his friend.
I also want to accurately portray the breakdown, and I am also not that great at writing. I know one of the rules for breakdowns is "show, don't tell" and to perform research on the aforementioned subject.
So if anyone has any tips on what to do or how to write such a scene, your input will be valuable.
Edit: The main character suffered a temporary panic attack when they heard that their best friend was dead and shut themselves out for a while. They were the only friend they ever had.