This sets up the remainder of the plot by explaining how he comes to meet Harry and the others and why it is crucial that he gets the money to move elsewhere: not just because he is presently homeless, but because he needs to get away from the violent rival he is feuding with. The newborn son and difficulty he would face getting a job with his record – in particular scenes where his girlfriend and Harry put pressure on him to do the right thing for his family – set up clearly his motivation for attempting the whisky heist. There is really no gap in the logic.

You also seem to miss that the theft of whisky from the barrel is not just considered minor because it is whisky, but because it is small enough in quantity to be put down as the "angel's share", another point deftly introduced earlier in the film.

One of the most striking things about this film is that, compared to equivalent "feel good" films, it comes across as incredibly realistic – partly due to the acting and the believable characters, partly as a result of the tightly constructed plot!