Marcello had a legitimate grudge against the Kennedy Administration. Like other Mafiosi, he had entered the US illegally through Tunis, which had a substantial Sicilian population decades ago. He had enough money and legal muscle to avoid deportation--all he had to do was show up at the New Orleans INS office quarterly with his lawyer. But in his first visit in '61 (first during JFK presidency), he was met by Federal marshals. They found that he had a phony Guatemalan passport, so they "deported" him to Guatemala by handcuffing him and flying him to Guatemala City in a gov't plane. The Guatemalan gov't declared him "persona non grata" and "deported" him to El Salvador. They did the same, and dumped him in a jungle just over the border in Honduras, where he was expected to die. He somehow traversed 10 miles of jungle and found a village, and eventually got back to the US. As soon as he did, the gov't charged him with evading $850k in taxes, entering the US illegally, and more. So, Marcello had motivation for revenge.

But, motivation alone doesn't prove that he arranged JFK's assassination. Other mobsters had motivation. So did Castro. So did others. But there's no proof that any of them did it.

I want to be very careful in what I say: The available, verifiable evidence does not support a conspiracy in the JFK assassination. But, it's what we don't know and probably never will know due to the passage of time--that stops me short of saying the case is closed.