Somebody is selling this idea.
Key words in Bowser's post is "I've read speculation."
If you track this back, it probably started with Nobel Prize winner Doris Lessing. Given some of her other opinions, I'm surprised anyone rational took this one seriously.
It's been picked up, mostly by pro-Obama types, in an effort to raise racist speculations, play into the usual silly stereotypes of the Right, and to paint Obama at the new Robert Kennedy.
As far as Bowser's theories go, it's worth remembering that JFK and RFK were not killed by rednecks.