Fear

Young left-wing supporters of multiculturalism were gunned down in Norway because of fear.

Before this: an attack on members of the Unitarian Church in Knoxville. Attempted murder of Gabrielle Giffords.

Fear creates violence. But fear is a powerful political tool.

My comment after Knoxville:

It seems to me that this is the inevitable result of a media environment in which it is okay to joke about assassinating a liberal candidate for the presidency, in which an extreme bigot is called kind and decent by the President and venerated in the media after his death, where a high-profile media figure explicitly identifies liberals as internal enemies, where countless slurs and attacks on left-wing views are broadcast and repeated daily.
There is a huge media machine working feverishly to create hatred towards liberals. How then can this violence really be any kind of surprise?

But there is no responsibility taken. (Edit to add: More.)