I’ve been giving these questions some thought over the last thirteen years.
Don’t overthink Johnny Boy. He’s a supporting character in what is Charlie’s story. Johnny Boy is somebody that Charlie feels an obligation to even though he knows he’s causing him problems. Johnny Boy is essentially the personification of the neighborhood. That sense of obligation to where he came from is why Charlie can’t solve his problems by just walking away from it all.
Like Johnny Boy, Travis Bickle is intended as a personification rather than a full character; in this case, the personification of a point of view. Travis represents the way some people see the world, which means that not everything we see on the screen is an objective portrayal of what’s happening. It’s events the way Travis sees them happening. And the ending, as others have said, represents the point (maybe in death) where Travis’ world separates completely from the real world.
Here we have a full character. Jake LaMotta does not represent anything other than himself. The point of this story is to show how even a man with great gifts can be brought down by the flaws that exist alongside those gifts. The message is that you can escape a lot of things in life but you can’t escape yourself.