Yeah... we have a great deal of human history more or less detailing how terrifyingly easy it is to get sane people to commit atrocities. Yes, a murder spree is generally what you'd consider
evidence for mental illness, but it's not proof
of it, and there's a lot of cases -- such as, to all appearances, the case being spoken of -- where the person committing the act is not mentally ill. The situation they're in just such that they're able and willing to perform terrible acts, often on people who are to outside observers entirely undeserving of such things. Sometimes they're just terrible people, sometimes they're just in a terrible situation. Sometimes something in between. That doesn't make them insane.
To an extent, it's almost unfortunate. Mental illness is something often identifiable and sometimes treatable, or at least able to be largely mitigated in ill effects. Baseline human psychology is... less addressable. We
want people like Roof to be insane, because that means we don't have to address the conditions that led to his actions. We just have to fix the person, rather than attend and be aware of the fact that their actions are a potential weakness for
anyone, not just the mentally ill. There but for the grace of god, as the saying goes