Well, like I say, pure speculation, but it fits and makes a kind of internal sense.
If the shooter was operating from a really flawed view of reality, like actually believing that the world was about to end, and end horribly, and also if that person believed that dead people were going to "a better place", then this would be a logical thing that any good person would feel duty-bound to do.
Yes, you and I know better, and we would have reached a different decision in the same circumstances, we hope, but you and I are more-or-less mentally normal.