Groups attempting to grow in membership and power promote evil by treating members of other groups as objects rather than people. The same can be said for individuals, exemplified by mass shootings such as the recent one at Virginia Tech.
When one person kills a group of strangers, the killer has objectified the victims: they represent a set of characteristics that the killer feels must be destroyed. The victims are perceived as either a threat, or an obstacle to what the killer believes is good.
The security of potential victims depends on identifying those who perceive them in these ways, and then applying the most practical and appropriate security approach or approaches (offense, defense, containment, alliance, assimilation, or retreat). Western societies like the United States typically apply defense (use of force by police), containment (prison, weapons controls) and assimilation (education) to minimize the threat from individuals. Only a few societies, considered barbaric by the rest the world, also use offense (the death penalty). Few others will use alliance (such as corruption and the support of terrorist organizations). Retreat, or surrender, is seldom an option.