In the comic books Joe Chill was a ruthless and cold killer, who came to kill the Waynes cause Thomas was planning to testify againts some mob boss. Honestly, Joe Chill was almost exactly like the guy who was portrayed in Burton's Batman, when Bruce remembers his past. That was the comic book Joe Chill, not a shaky bum who wanted to steal some money for more crack. At least that's what it was originally, maybe they retconed something laterly, I am not really up to date with the comic books... the point is Nolan's Chill WAS a liberty. It worked for the movie but it was still a change from the comics. The very first original that is.
Actually, the characterization of Joe Chill is vague and inconsistent at best.
In the original, very brief origin of Batman (in Detective #33), Chill says about as much as he does in
Begins. "Give me her pearls," etc. He does seem to be more vicious than in
Begins, saying something along the lines of "This'll shut you up!" before shooting.
But in the issue where Chill is killed by the mob for being "the man who created Batman" (Batman #48), Chill is clearly shaken up by young Bruce, yelling "Stop looking at me like that, kid!"
Begins doesn't emphasize this penetrative, accusing stare of Bruce's, but Nolan's Chill seems awkward and uncomfortable enough just the same.
I chalk it up to the differences between film and (especially early) comic writing. People act and say things in comics in a way that people simply don't act in real life. This is not to disparage writers like Bill Finger and Gardner Fox, but writing was very simplistic back then. Characterizations like this have to be revamped for the sake of, well..not coming off as a stilted, live action cartoon.
And of course, it's also a creative decision used to serve the story. Without Chill being a "desperate man", you lose that whole theme of poverty, crime, corruption, the motivations for Ra's, etc that's so central to the film.