The Assassin is a character created by author Peter McCurtin, who wrote a lot of books. Besides three (at least) Assassin novels, he also wrote for the Marksman, Carmody and Soldier of Fortune paperback series, as well as a ton of westerns, crime dramas and even the novelization of James Glickenhaus' screenplay for the exploitation sickie THE EXTERMINATOR (I'd love to read that one). McCurtin doesn't seem to be very good, despite his busy resume. He has a blunt writing style…well, maybe "style" isn't the right word. He writes as though his routine is to sit down at his typewriter and punch keys until he hits his word count.
MANHATTAN MASSACRE, published in 1973, reads as though it could have been the Marksman's origin story, and Dell's attempt to copy the success of the Executioner and other similar paperback action heroes is obvious. Robert Briganti is a crack shot and gun dealer whose family is murdered as payback after he refuses to arm a Brooklyn mobster named Joe Coraldi. Feeling he has no reason to live, except to get revenge, Briganti rips off some weapons from the munitions company he used to work for, and sets to work knocking off Coraldi and his men. He makes audio tapes, describing his bloody mission, and sends them to the media and the FBI, hoping to make the public aware of how evil the Mafia really is.
It's really a pretty decent read, even though there isn't anything unusual or original about it. MANHATTAN MASSACRE even has a title reminiscent of an Executioner, Death Merchant or Butcher title, which is certainly no coincidence.