This article might be correct for the columbine massacre, but I don't think killing 20th kids has anything to do with a revenge for being bullied.
I often see in myself the capability of performing such an awful act as Adam did. Not for any particular reason, but simply because I've bottled up immense rage over the years; rage that I somehow manage to keep under tight control. A so-called "Irish temper," if you will.
Honestly, I find it surprising that things like this don't happen more often among us "nerds;" that more of us don't lash out at society.