I think they could actually make Nightmare not only less ableist but way more interesting of a character if they literally just switched which of the personalities is the "original."
So to greatly oversimplify, DID is in essence a way for a person's mind to cope with different stressors by creating alters that can better handle those stressors. So a person cannot form an alter that deals with stress by acting violently unless they were already the kind of person to commonly use violence to deal with stress before forming that alter. As such, "innocent person develops evil murderous second personality" is not really a thing that happens, despite what mainstream media would have you believe.
However, a violent and cruel person developing an alter that's soft-spoken and timid is not only feasible, but VERY interesting as a concept. It creates very interesting implications about the character, especially if the timid alter is the one fronting most of the time. It also reinforces the subtheme that no one is truly, completely evil, and that greed and hatred are what create the worst parts of the world. Plus, having a character who is evil by choice instead of because of an ableist depiction of a mental condition is far more fun
