It is not uncommon for musicians to use drugs, for while they are under the influence of drugs, they are often extremely creative. In addition, the lifestyle of a musician makes the use of drugs tempting as they are often up long hours and/or odd hours and taking something helps them perform longer--at least they think so.
Musicians and other artistic types "march to a different drummer" from the rest of humanity. Extremely sensitive, the world often becomes more than they can handle and they seek outlets for their storms from within. Sonny says that he has done heroin because he felt in control when he did. At any rate, the nature of a musician is no commonplace one.
I think #2 makes a number of extremely valid points. The whole nature/nurture issue is one that has troubled academics for centuries, and I think the author of this short story is very clever to ensure that there is evidence for both. In a sense, we are unable to tell if we are more a product of our genes than our background - both will clearly play an important role in establishing who we are, but for some, arguably, nature will take the bigger role and for others nurture.
I am offering only an opinion because there is evidence in the text to support both points of view. Implicit in the story is that Sonny has had a problem with addiction and a gift of music since he was young, and that the narrator has always trod a "straight" path. If Sonny was born with a predisposition to drugs and alcohol, then certainly, that predisposition is "nature." But we do not know if there is a predisposition in this family, which would be a strong argument for "nature," too. On the other hand, it is clear that both brothers have grown up in an environment in which drugs and alcohol are easily available and an environment of poverty and, for many, despair. That argues for "nurture" as the cause of Sonny's problem.
Now, having pointed out bases for both points of view, it is my opinion that given our current understanding of human genetic and behavior, this is a false dichotomy, meaning there is no reason to say anyone is the result of just one and not the other. People are born with strengths and weaknesses, for sure, but whether these strengths and weaknesses emerge, or the degree to which they emerge, is a function of environment. We are creatures of both, so the idea of choosing seems to me to be a mistake. This is a complex area of debate, and you will certainly find people who disagree with my position.