Paul Lazzaro is the fake name of a real guy the narrator mentions in the first chapter, who "really did threaten to have his personal enemies killed by hired gunmen after the war" (1.1.1). Lazzaro is a fellow American POW with a grudge against Billy because he believes it's his fault that Roland Weary dies of gangrene.
(Lazzaro needs a crash course in "So it goes.")
Of course, Lazzaro had not known Weary before meeting him on the train from Luxembourg to Germany, where Weary dies. Lazzaro's insane, murderous loyalty to Weary's memory doesn't really have anything to do with Weary. Lazzaro is a guy with a permanent chip on his shoulder, a total psychopath who judges every encounter he has with a person based on how much respect they show him. Revenge is the sweetest thing in life for Lazzaro, and he is always looking for an excuse to get some. The narrator compares Lazzaro to a rabid dog (6.9.3)
Lazzaro's threats seem to work, because Billy really does believe that he is going to succeed in shooting him out of the blue. That is how Billy sees himself dying, and he has even seen the date: February 13, 1976. We cannot help but notice that February 13, the date of Billy's projected death, is the same day as the Dresden firebombing. We're just sayin'.
Anyway, there is no way to prevent or change this future assassination. Remember, according to the Tralfamadorians, we have no free will. Check out the Tralfamadorians' "Character Analysis" and our theme on "Fate and Free Will" for more on this point.