If there is anything to blame it is the increase in "reality" television and the advent of the 24/7 news cycle that gives the killer immediate fame.  They see the previous killers get all this grand publicity and they want it for themselves even if they won't be around to enjoy it.  They want to be infamous.  They want to one up the previous kill number.   You can expect to see the next mass shooter going for a hundred kills.  When they can't do it just with guns they'll look to use something bigger..... grenade launchers or bombs.