I'd like to ask exactly HOW someone ISN'T mentally ill if they decide to shoot up a school??

People claim "there's no evidence for psychological or mental health issues affecting the shootings" but i don't know how rational of a conclusion that claim really is.

I mean, I'd really like to know what the other reasons are for why someone would do this. Excluding the terrorists known commonly as incels ("I'm going to shoot a school full of my classmates because a girl I liked didn't accept my advances"), there have been individuals that have literally shot up classrooms where only kindergarten kids were.

I don't agree that a shooting like the Sandy Hook shooting was on the same level as that of an incel killer. The man was a grown adult and his victims were all 5-6 year old children, and I'm really wondering how exactly someone didn't rule out mental health in that case. Because if you wake up in the morning and decide to shoot up a classroom filled with kindergarten kids, you are mentally unhealthy to some extent.