I know it's just a movie, but ...

I always wonder in one of these drug movies, why does the drug
deal continue to sell drugs to someone who cannot pay for them?
He did not really seem like he enjoyed beating Cathal up, although
there were a lot of just plain mean people in the movie that the movie
depends on being just mean. Is that one dimensional of the movie,
or are there people like that? And then ... why kill him or make him
so wounded that he cannot recover and even steal money to pay off
his debt?