The actual Forensic evidence proves Sirhan couldn't have fired the shots that killed RFK.
It's not speculation that there was more than one shooter. There's proof.
Prosecutors withheld the autopsy report from Sirhan’s defense lawyers until six weeks into the trial, showing that Kennedy had been shot at point-blank range from behind.
https://www.washingtonpost.com/news/true-crime/wp/2018/06/05/did-l-a-police-and-prosecutors-bungle-the-bobby-kennedy-assassination/
He also couldn't have fired more than 8 shots yet there's evidence that as many as 13 shots were fired...
RFK was not hit in the head by the first bullet. Paul Schrade was. And he has always maintained that he was shot by Sirhan. Lately, Schrade has come to believe that there was another shooter who killed RFK. I am not sure what that is based on but if it is based on what happened after the first shot, Schrade was not in the best position to know. He was hit in the head by the first shot and immediately collapsed on the floor.
The idea that Sirhan who approached RFK from the front could not have shot fired the bullet that entered behind RFK's right ear is not very persuasive. There was a lot of commotion when he started firing and heads and bodies no doubt changed relative positions and orientations. Sirhan unloaded his .22 handgun injuring several others as well as RFK.
No one can make a credible claim that Sirhan did not fire shots. But it has been suggested that he may have been hypnotized by his conspirators. I don't doubt that Sirhan would have been in a bit of a detached mental state - that would almost be necessary in order for him to have done what he admitted to doing. But to assert without evidence that he was hypnotized and programmed to do the assassination is fanciful.