I would assume you are referring to the school shooting. But you used the word "evil" which to me, means war, genocide etc. Not that I think those things are always bad from a practical point of view. For example, from a standpoint of human behavior, I don't think it's unreasonable for Palestinians to try to destroy Israel and I don't think its unreasonable for Israel to try to destroy Palestinian controlled areas. In that process, schools tend to get bombed, buses blown up etc. Not that its "OK", i just mean it is to be expected. A funeral in Pakistan was bombed by US drones to take out 2 people, killing another 15. Is that "evil" - I suppose. Depends on which side of the act you are on.
My point was that you are ignoring human nature. Its mans nature to always want more, to strike out for more, to take more and to gain more status, more power and so on. It's an evolutionary trait and natural behavior. The vast majority of "evil" are people will less, trying to get more. Greater evils like Stalin or Hitler or broken people doing the same thing for the same reasons, just to an unreasonable degree. As people, we have a strong tendency to misunderstand why people commit acts of violence on a large scale. Al Queda for example is not on a holy quest. Religion is just a convenient rallying cry to find people to go out and die so others can gain power and status. The organization are simply nothing more than a bunch of angry people, from a region that breeds angry people and who have nothing (namely status and power), who find increased status and power in their actions. This is why leadership are never the ones strapping on the suicide vests in spite of all their professed beliefs and this is why leadership is always far from the fight.
When it comes to social hierarchy, those on the bottom want to be on the top. Eventually leadership will form and organize people to try to make that move.
On a small scale such as the act of 1 or 2 individuals, I don't think its fair to group with the Stalins or Pol Pots of the world. Different acts, on different scales, different intent and with different motivations.
"war is the natural state of man..."