JFK, Dr. King and RFK are the subject of so much controversy is that, if you look at the list of political assassinations in the history of the United States and, indeed, many of the assassinations of leaders in other countries, the assassin practically always either has a personal motive or is acting as a part of a conspiracy or at least out of political fanaticism. Just look down this list. You will find the United States at the bottom of the list of assassinations on the American continent.
http://en.wikipedia.org/wiki/List_of_assassinated_peopleOur doubts are not inconsistent with the "means, motive, opportunity" test which is applied to persuade juries of guilt in criminal trials. Our minds tend to want to be satisfied that these a killer or assassin had a motive strong enough to make his or her guilt believable.
http://en.wikipedia.org/wiki/Opportunity_(law)
Yet we are supposed to accept the idea that the assassins of JFK and Dr. King had no personal motive and no affiliation with some group or government entity that at least advocated political violence.
The case of RFK is a little more difficult. Sirhan may have been motivated by Palestinian nationalism. I am unaware of any evidence that Sirhan belonged to or associated with Palestinian groups that advocated the use of violence against American leaders such as RFK. If Sirhan did not have ties to such a group, if he was a lone assassin, he was a very rare type -- the lone, insane assassin.
If you look at the assassinations of American leaders as linked from the website above, and I qualify this with the caveat that I have not made a statistical analysis or delved deeply into the details of all of the assassinations beyond those discussed in the links from that website, assassinations attempted by lone madmen are rarely successful. Nearly always, successful assassins either have a personal ground for the assassination or are members of conspiracies, or are actively part of or at least associated with a group that encourages fanatical hatred.
No wonder we tend to feel uncomfortable with the official stories that Lee Harvey Oswald and James Earl Ray were assassins. Where are the motives strong enough to make them risk an assassination attempt? There isn't much evidence that they were truly insane. Also, they did not use the methods most often used by insane, lone assassins. They killed their victims from afar. Insane, lone assassins or would-be assassins most often rush up and attack their victim or would-be victim from a short distance. Ray and Oswald just don't fit the picture.
And since the explanations of the assassinations of Dr. King and JFK do not seem credible, we also doubt the official story about the RFK assassination. It's just natural for us to feel dissatisfied. There have been some amazing cover-ups of conspiracies.
Read about the death of Crown Prince Rudolph of Austria. It is still unclear whether he was assassinated or committed suicide after many, many years. The Emperor of Austria, Franz Josef was very successful in covering up the details of his death.