“My stepfather has become increasing verbally abusive to my mother, who only tries to please him. He has always had a reactive streak in him, and calmed down a few years ago, but with the increase of success within his business he has become far worse than I ever remember. I often overhear screaming fights where he lashes out at her, and tells her she is an idiot, has the brain capacity of a gnat and how he is the best she will ever find. When it comes to affection, he only shows me anything when there are other people around, and I have come to the realization that he does it purely as a front. He often states how much people admire him for taking on a family but with his character I doubt he would have a family without us. He emotionally abused my brother so much that he eventually left to live overseas to gain some peace and perspective. I am studying so listening to their fights is part of the package.” (Edited)
Abusive behavior will prevail and grow only if accommodated. Your stepfather has no incentive to change while his behavior is tolerated. You are an adult: remove yourself from his environment and hope your mother does the same.