I had an autistic cousin. I inherited numerous photographs from various relatives and a couple of years ago I found some of him and his brother with me and my brother all of us within a year of each other in age. Jonny was about five in the photograph. We never saw him at all after that, not even his brother did. He could not be trusted around people. He died this February in an institution at the age of 68.
I’m sorry about how life turned out for your cousin.
Sometimes there is no “good” place for people, especially those who have proven capable of extreme violence. A family unit has to act to protect and maintain itself vs having everyone”s actions and lifestyle ruled by the unmanageable behavior of one individual.