There's also explosives, home-made or otherwise, and more mundane "domestic terrorism" that goes after food (I mean, grocery stores just put out
all the food on easy-access display..), city water supplies, electricity, gas lines... It's fortunate that there are very few real psychotics who want to cause a lot of damage, and also that of those few only a few again are intelligent or imaginative. People have a good reason to suspect a lack of imagination of imaginary killers: real ones don't seem to have much of one.
What intrigues me currently about Holmes is that his residence was booby trapped sufficiently that federal experts had to get involved, so he's more capable than the average spree killer. Yet he alerted the authorities about them, he didn't let them "figure it out" with the death of some cops. I don't see any sort of twisted logic in that. But I initially approach this topic with a gamer's "high score" mentality such that even Brevik's count seems small, the behaviors of killers are thus constantly perplexing from that angle.