I agree. We're all vulnerable.
But I thought the study relevant in this particular case since the perp is specifically blaming the vaccine for his depression. He was, perhaps, more vulnerable to misinformation to begin with, due to depression. Then it all got muddled and reinforced in his head.
And my personal take on it, misinformation is addicting. Once people get hooked on misinformation, they want more of it and enjoy spreading it, even if it alienates friends and family. Not a proven fact, just what I've noticed. And they get angry, like an addict, when questioned, imo.
The neighbor's comments that it became like a faith to the shooter rings true, imo. Misinformation becomes some people's personality, imo.
jmopinion