Don't understand what you're saying with the bolded. If someone is one step away from killing other human beings with firearms for any reason, that's mental illness. It is not normal to feel that way constantly, it is not normal for people to despise random human beings so much they feel the need to kill them.
Mental illness is underdiagnosed, people don't want to admit they're having difficulty with handling life because of the stigma regarding mental illness. No one is being disregarded, its about accepting the fact that people are born with different brain and personality features.
It's not to say that there's no mental illness involved. What I mean is, if you disregard killers as just "those with mental illnesses," you're basically resigning yourself to never fixing the problem. "Oh well of course he killed those people; he had a mental illness!" I don't believe that's the right way to look at the issue. That ignores all other causes. What if a kid got bullied everyday and after months of noone caring at all, he finally snaps and shoots up a school. Is the problem that he had a "mental illness"? He might have one, but he probably wouldn't have shot up the school if he wasn't bullied everyday.
Now it's obvious from our perspective that this guy had some issues. However, we should strive to understand why he had those issues and whether this all could have been prevented.