Salvador Allende in Chile comes to mind. I think the head of Congo/Zaire before Mobutu met a similar fate. Sukarno in Indonesia, but I don't know if the U.S. played as big a role there.

Of course an American might not have actually pulled the trigger, but the U.S. was a/the primary mover in many of these situations. IIRC, President Carter issued an executive order banning the assassination of foreign leaders.