He is called samurai but he is really a ninja.
Samurai is "one who serves". Service is rendered to some sort of lord (Daimyo) or authority figure. A samurai without a post is a ronin. Jef Costello is a free lance hired killer without a bond to anyone in particular, in fact his non-association in an essential element because he has no connection to the victim to trace back to his employer. (Just what Yabu was counting on in Shogun as well.) I think that makes him a ninja. Notwithstanding he shares the code, he is a free agent not a server. Of course, that would be a terrible title today because of all the silly movies using that word, Teenage Mutant Ninja Turtles, Surf Ninjas, and endless action movies and dumb comedies with Ninja in the title. It would not have been a familiar word in 1967, I don't see movie listings with the word until the 1980's.
https://en.wikipedia.org/wiki/Samurai
https://en.wikipedia.org/wiki/Ninja
The only serious defect in this movie is how Jef sticks out like a sore thumb at the night club in that film noir beige trenchcoat and hat outfit in the midst of black evening wear. Apparently despite the meticulous attention to disguising the stolen car, getting a disposable untraceable gun, arranging two distinct interlocking alibis, using and discarding the white gloves, the idea of camouflage at the target site somehow entirely escapes him.
CB
Good Times, Noodle Salad