I had a verbally abusive alter. My t told me they all have a purpose. I eventually learned she thought she was protecting the system by her abuse. She wanted to keep the system secret by keeping us from communicating, so she would bully to shut things down. She thought if one of the little ones was allowed to interact with the others, everything would fall apart and outsiders would think we were crazy. She did change. Now she is protective in a better way.
I had another who was downright sadistic and manipulative. He acted this way because it was a way of relaxing (I know that doesn’t make sense but go with it) and it was really his only way of interacting with others in a satisfying way- like, his way of enjoying someone’s company. Even he changed, and became one of the most helpful alters in the system.
So yeah, it sounds insane to suggest that a malicious alter is serving some purpose but, at least in my system, I’ve found out that I can’t get anywhere with them until I respectfully acknowledge what it is they are trying to achieve. After that there is some hope that they will listen to me, and I can suggest different ways of accomplishing that purpose that are better for us all. Just like dealing with difficult people in life. They aren’t willing to hear what I have to say until I’m willing to meet them where they are at and hear what they have to say. I don’t have to agree, wrong is still wrong. On the outside we usually have the option of avoiding awful people, but with insiders we don’t. It takes a certain patience and respect.