When Ray and Dave go and do over that drug dealer, I was just wodnering why?
The obvious reason would be to get his money but they take his drugs as well and i can't see either of them using or selling drugs.
As I Hustle In The City, For A Paper Stack, Lord Knows I Got The Devil On My Back
One word : greed.
It sets up Ray and Dave not just as criminals, but the uncaring nature of capitalist society. Note they are impersonating the police, representatives of the State. There are no half-measures, there is no generosity to be seen - they ruthlessly take it all even if they have no use for it, just because they can. Greed at its worst.
All through the film any time anyone acts selfishly they get punished (beaten, shot) - any time they look out for someone else, they are rewarded (Connie saves the day, Stevie never abandons Ray).
Even in the simplest of moments like when Phil Davis is explaining how to keep hold of a woman with kindness, the film is waving the socialist banner. Look out for each other and survive, act with self-interest at your peril.
Also, this first scene is there as it explains the scene later on in the movie (at the police station) why and how Ray has a police badge!
It's just a scene set up to explain a later, more important scene.