Bigotry is not a mental illness, and actually mentally ill people are much more likely to be the victims of violence than to perpetrate it.
Please stop spreading this ableist excuse.
This guy killed someone because he was a violent hateful bigot, and is direct and not at all unexpected result of a violent hateful and bigoted society. That might not be an easy or comfortable reality to deal with, but that's what it is. Stop scapegoating mentally ill people (who yes, deserve significantly more help and support than we get, but that doesn't change the fact that society created this, and many other, killers, not mental illness).