I think, though the phrasing is awkward, that he is saying the majority of them are indeed for revenge. For some reason also saying "the very few" in there to imply there aren't actually many school shootings at all. Which I suppose is one of those "relative to what?" questions because I'd say there's been loads of them in the last decade.
Yes, you are correct, I am saying that the majority appear to be revenge related, and the total is not very many, which of course is a good thing. Yes this is just my own personal opinion based on reading everything I could find on the subject, not only from our own local news sources, but also international web news, psychological analysis of professionals, and some other information sources I have.
I agree that I am not an expert in this area, even if I think I know what I am talking about.