Terror and Identity
The Boston marathon murderer allegedly left a message in the boat where he hid saying that his attack was in response to the war in Afghanistan. "An attack on any Muslim is an attack on all," he proclaimed. This notion of group solidarity is at the heart of the idea of gang, tribe, clan, and probably religion as well. This statement has sort of mirror image - "an attack by any Muslim is an attack by all."
The logical conclusion of this reasoning is war until the extermination of one group or another - or perhaps all groups but one. The evidence suggests that throughout most of human history war has been a major cause of human mortality, and that we evolved to compete in small groups with each other. That instinct to organize against each other is at the center of racial, religious, and national strife as well as the extremes of gang warfare and political partisanship.
If you sell an LED light as cheaper than its incandescent counterpart, both conservatives and liberals are more likely to buy it. If you add that it also helps conserve our resources liberals like it better and conservative like it less. If you give some teen aged summer campers red baseball caps and the rest blue ones, they will quickly evolve into warring tribes.
Civilization depends on being able to tame the tribal (or clan) monster within. The trouble is that it doesn't take very many Tsarnaevs too set off a fatal religious war.