If you consider someone an enemy, why would you even consider stopping their assassination? Unless, of course you are a committed Christian.
The Castro regime, specifically Fidel, for the numerous assassination attempts on him. The Kennedy brothers were absolutely determined to eliminate Fidel because of the humiliation of the Bay of Pigs. This was a personal issue to the Kennedy brothers as no one got away with beating let alone ket alone humiliating a Kennedy. There were several real ambuscade shoot out type attempts on Castro, which leaked into the press at the time but have never appeared in later print accounts. Castro, tyrant he is, had a valid feeling is was either that he eliminated JFK or JFK would eliminate him.
So called organized crime or specifically the ‘west side outfit’ from Chicago.
Joseph Kennedy, Sr. Had made a deal with these people before the 1960 election that in return for their complete support in producing the vote in Chicago there would be no undue federal criminal investigatory activities. Unfortunately JSK, Sr suffered a massive incapacitating stroke in Dec 1961 and there was no one to effectively reign in RFK’s anti-racketeering crusade vis-a-vis the Chicago gangsters. To men such as Sam Giancana it was felt they had been double crossed. There is only one way to respond. Strike at the head if the family that had betrayed them, i.e. JFK.
Third is the Ngo family as payback for the events of 1 Nov 1963 in Saigon.