I think the key point here that was only touched on a little is that these tend to be realizations of fantasies. It’s difficult to understand the mentality, because for most of us, we don’t have fantasies that include killing a bunch of people. We’re talking about the sort of people who are damaged enough that they want to exact revenge on people, or that they don’t even really see others as people. But maybe if we imagine the same sort of passion with which many of us might pursue our own fantasies, it seems to make sense in a sick way.
That is, we’re talking about people where this fantasy has so consumed them to the point that they’ve spent the time any energy to plan and prepare for it. At the end of The Princess Bride, Inigo Montoya has spent 20 years planning his revenge, training in sword play, even to the point that he knows exactly what to say. But in that moment, he just says that same phrase over and over again, and almost fails. And even though he eventually succeeds, he’s left wondering what to do with his life now that his whole purpose in living is fulfilled. In that sense, I could see many of the suicides just being messing up or failing, or in the heat of passion, seeing their whole meaning in life fulfilled, see no reason to carry on.
I could also see, not just the idea of not wanting to face the consequences, but after realizing their fantasy, to some extent coming to grips with the reality of the situation. Sometimes, perhaps most times, when we actually get what we want and have fantasized about, we’re disappointed, because the anticipation is always more than the reality. The difference is, in this case, we’re talking about a gruesome fantasy that I imagine would weigh on just about anyone who has even a shred of a conscience.
All in all, I doubt it’s typically planned to kill X number of people then off themselves, but just that they do it, and realize it’s over, and it’s that last bit of control, the gravity, the fulfillment, or whatever.