Mental illness is something that's been a huge interest to me and something I wanted to add to my characters because I myself deal with mental illness and wanted to feel less isolated. So much so that I have been taking psychology classes and doing way too much research than what is probably necessary for characters and rp. I'm currently in my Abnormal Psychology class, which while I am not learning as much as I was hoping for due to the pandemic screwing up classes, I am still learning new things.
I see nothing wrong with giving characters a disorder or disability when done right and for the right reasons, it adds depth, explains unusual behavior and gives them something to grow from. I'm going to use my comfort character, Dylan as an example.
My character Dylan is who became the one I gave a mental disorder to, to explain his actions and behavior and to give him something learn and grow from. He has Antisocial Personality Disorder along with Post-Traumatic Stress Disorder. I rarely use the term sociopath due to it's infamous use in movies but that is what ASPD is in a nutshell, just not exactly how Hollywood likes to depict it. When it comes to Dylan, he is not a psychopath, I don't like how people so readily use the term to describe any violent person, because that's not what a psychopath is. If you want a psychopath, go meet Kevin, he is my only true psychopathic character. Dylan is a murderer, but in the world he lives in, it's not really surprising. He's cynical and callous, distrusting of others and feels wronged by the people around him because of what he was put through. He is damaged and broken down and at the end of his rope. Dylan is the side effect of the world he lives in, someone who was seen as lesser than those around him and treated as such and as a result, he fought back violently. But despite this, he is receiving help, he is being encouraged to move on with his life and not let the past control him. He struggles and backslides and rarely sees the light at the end of the tunnel, but he does want to better himself, and he finally has people in his life willing to stand by his side and pick him up whenever he falls.