I haven't been paying super close attention in here, but to me it feels like terrorism is when someone threatens to, attempts to, or does kill a bunch of people.

The FBI defines it one way, Michigan defines it in a way that they can charge this guy. I think it fits the bill. We don't know his motivation, so it's pointless to argue over definitions that are based in that.

In my opinion every mass killer has the intent to influence society or the gov.

Even if it's just to "go out with a bang" and get attention for themselves before they pop their cap off or spend life in jail, there are ways to do that that don't involve killing lots of people. They want people to be scared, no two ways about it. Scared. Fear. Terror......ism. Even if their motivation is something stupid like incel stuff, the intent is to teach a lesson.

"I'll show them!" is the mantra of the mass killer. They usually have something to show, and that's the motivation that fills in the blank to make it terrorism IMHO.