No. Racism, hatred, violence, etc are not a mental illness. They are just that- racism, hatred, violence, etc.
If you have been indoctrinated into thinking that there are people who don’t deserve to live, the extreme end of racism, then killing someone seems perfectly normal.
I am currently working with letters and other documents between politicians, doctors, social workers, etc from the way too late into the 20th century where they’re discussing openly how to make euthanasia of “defectives” legal and practice it on a large scale. Were these people mentally ill? I won’t retroactively diagnosis someone but no, they were just convinced that certain people didn’t deserve to live.
I think that’s where Dylann’s mind was. They don’t deserve to live.