Nobody can speak for victims or suspects except for themselves, but when people talk about massacres like that, people almost always cite one of two reasons, catharsis (think the ones talked about on the psychology programs) and philosophy (think of your stereotypical hate criminal in this case).

I refuse to be a judge on whether these are true, but in the case of the first one, if true, just be there for people. Perhaps you believe in the butterfly effect, in which case choosing specifically downtrodden people as friends is akin to saving hundreds of lives. Check on your neighbors if you must, give to the fatherless and the unlucky, provide compliments, hear the unheard, forgive the unforgiven, be a guardian angel, etc. It is true that one thing that always strikes me when this kind of shooter happens is how clique-ish and honor-obsessed their communities are. I have friends in New York and they often talk about how Upstate has very few shooters compared to Downstate and many cite that Upstaters act very quasi-Canadian while NYC is the city from Judge Dredd by comparison.

On the other hand, hypothetically, as for the other kind of mass killer, can you really help them? If it's as simple as that, I would liken them to people innovatively taking their liberties. Be peaceful but challenge their ideas and keep notes on who is who that can be added to a collection.