SWOT, yes, I remember your story from previous posts. So, so sad and tragic about your adopted son. I work for a psychiatrist who deals with troubled kids and teens, and I have learned that it doesn't take long for the human brain to be damaged by neglect and/or abuse. Reactive Attachment Disorder develops within the first five years of life.
Our oldest son, J, was adopted when he was four months old and had been placed for adoption by his birth mom when he was five weeks old. He spent about two months in a Christian family who fostered him until we brought him home. It was the happiest day of our lives, and he was a wonderful child. Anyway, that's water under the bridge. I don't think he has Reactive Attachment Disorder (RAD) but his birth mom told me a year ago that she was Bipolar. I didn't have the presence of mind at the time to ask her if she had been formally diagnosed, and she died in a car accident a month after I spoke with her so I will never know that. But there's a strong chance that is what has happened to J. I always wonder just how much of his mean, disrespectful, abusive speech is his "illness" and how much is just something he chooses to do. That's something I really need help in understanding. How much responsibility does his illness get for his behavior? Still working on that one.
To answer your question, yes, we have a younger, biological son. He was just here for Christmas. Now that we have blocked J from our phone, we are wondering just who he will turn his rage and hatred against. We hope he will not turn it against his brother. We will see what happens.
By the way, I read your posts often and am helped by the thoughtful, wise counsel you offer. I may not always respond with a comment, but I read them, and I am helped by what you have experienced and the wisdom you have gained.