He wasn't a ninja, he was a ronin. Hence why the full title of the movie is "Ghost Dog: The Way of the Samurai". It also explains why the movie is interspersed with readings from Sun Tzu's "The Art of War".
I thought it was a good film, very philisophical. A lot of symbolism within it. The same director also did a movie called Dead Man with Johnny Depp, about an old west accountant who becomes an outlaw gunslinger. It has one of my favorite lines in there: "My name is William Blake; have you read my poetry?"
When you see that part of the movie (if you do) you'll know why.