Robert had even more problems than he did. There are recordings of Jimmy Hoffa and his associates plotting to shoot JFK and Robert or use plastic explosives to blow them up, while Carlos Marcello, the New Orleans Mafia boss, was seething mad when Robert Kennedy deported him to Guatemala.
He made his way back to the US--in a plane supposedly piloted by David Ferrie--vowing to take revenge for the disrespect that had been shown to him. He threatened to kill the Kennedy's more than once, but he was by no means the only one.
Ferrie later turned up again and again in Jim Garrison's investigations, since he could not even offer a satisfactory explanation of what he was doing in Texas on the day of JFK's assassination. There are people who think that he was even one of the shooters, but it's very hard to prove something like that.