1968 was the height of communist subversion in America. If Robert lived he almost certainly would have been elected and in my view his becoming President would amount to nothing less than a complete takeover of America by the radical left. Sirhan Sirhan was a psychotic jihadist assasin but he inadvertently helped America.
I don't see a problem with doing "what if?" historical fiction. This is not calling for assassination, nor rejoicing in it.
It's amazing, really. The Left's hold on the American consciousness is so tenacious that any criticism of their holy trinity - JFK, Bobby Kennedy, and MLK - isn't even tolerated by the Right.