Mailer thought that the CIA resisted any probing of its possible role in the assassination not because they were involved but because they couldn't be certain they weren't. In the early 1960s, the CIA was tied up with so many sketchy players and bad guys (certainly in the swirl of the mob, anti-Castro emigres, the Texas far-right and left-wing moles) that they couldn't be totally sure it didn't somehow connect back up to them. They didn't want to find out. Certainly they didn't want anyone else to find out.
Whether this was true as a factual matter or not I don't know. But as a theory it provided a plausible explanation of odd behavior, a shrewd take on human and bureaucratic nature while, all while making no outlandish factual assumptions.
An interesting 'innocent' explanation of Trump's behaviors with regards to the Russian hacking is similar.
Say you're Trump.
You have nothing to do with this. You know nothing about it. But think about all the crooks and gamers and sleazeballs around your campaign. There's Manafort, Stone, Page ... all their associates, not to mention your business associates with ties to Russian organized crime. (Stone publicly said he had some sort of a backchannel to Wikileaks.)
If you're Trump, how confident are you that a real investigation wouldn't turn up anything weird? Probably not very.