They're too comfortable in their lives to do the deed themselves, so they try to provoke the next Hinckley or Chapman.
Exactly!
Spot on! Some nut will say, "I saw the th sign. I saw the 'X' over Cheney..." And CNN will say, "Oooh. Well... We can't be responsible for every whack job that takes a flicker on the TV as a sign to go on a killing spree..."