I've noticed that the most well known shooters have a "gimmick"; Something interesting about them that can be said in a single sentence. The severity of the attacks is a very important factor (as seen with many of the people on this list), but a good gimmick can make up for a low kill count (see Brenda Spencer).
Charles Whitman: Sniper. Brain tumor. Killed mother/wife.
Brenda Spencer: "I Don't Like Mondays". Girl. Sniper.
Eric/Dylan: I don't even need to explain this one.
Seung Hui Cho: First major college shooting since Whitman.
James Holmes: Attacked movie theater.
Adam Lanza: Elementary school.
Elliot Rodger: Misogynist at a time when that was a hot button issue..
With the decreasing publicity of recent shootings, as well as the increasing prevalence/publicity of terror attacks, what do you think the next big killer will do? What will their gimmick be? Will there even be a next big killer?
"If opportunities for role fulfilment fall far short of the demand by those capable of filling roles, and having expectations to do so, only violence and disruption of social organization can follow. Individuals born under these circumstances will be so out of touch with reality as to be incapable even of alienation. Their most complex behaviors will become fragmented. Acquisition, creation and utilization of ideas appropriate for life in a post-industrial cultural-conceptual-technological society will have been blocked."
- John B. Calhoun