Ta-Nehisi raises what I consider to be a very reasonable point. Namely, that the man who shot MLK was egged on by public figures who demonized King as a communist, and that the McCain crowd is doing the same thing by calling Obama a terrorist-sympathizer.
In any group, there's an extreme minority that is prone to violence. Responsible leaders (MLK is a great example) do everything they can to rein these people in. Irresponsible leaders spout rhetoric that riles up their base, ignoring the fact that a few individuals will be inspired to actual violence. You want examples? Dave Neiwert's been talking about this stuff for years.
For the last 20 years, the right wing has been saying we're at war, that the enemy is less than human, and that the soul of our country is at stake. And those members of their flock that are already unhinged take them at their word and shoot up Unitarian churches, Democratic headquarters, synagogues, raves, gatherings of immigrants, and daycare centers that happen to be on the ground floor of a federal building in Oklahoma City.