I don't believe for a second that he didn't expect this event to trigger the reaction it did. I think this whole thing was a lefty setup for sparking a race war, somewhat similar to Charles Manson's "Helter Skelter".
Obama pals Dohrn and Ayers' Weather Underground and Manson basically had the same agenda: to incite a race-based, black vs white, revolutionary overthrow of the capitalist class.
Not saying he's as sick in the head as Manson. Only that the basic overall plan appears similar.
Not as sick perhaps, but just as cold blooded.