I remember reading that it was a rather shockingly violent film for the time. Obviously, it's a gangster movie, and a lot of other war movies and westerns were out then, so I always wondered why it was so 'shocking'.
Apparently it was the first film to show the actual result of being shot. As in, in one frame, no cut-away shot. Guy pulls trigger while other guy gets the blood spray. 'Back Then' it was always a quick cut away shot to show the victim gripping his gut and wincing. Once I realized that, yeah, it would be quite dramatic to witness that level of realism for the first time. Most of the other movies were violent, but yeah, guys just fall over, a lot less blood. It set of the grindhouse hyper-violent and gory films of the early 70's.
Anyway, great film. And, while not 100% accurate, I really how they did the ending.