Baldwin seems to feel a great deal of sympathy for Sonny. He doesn't paint him as a nameless, faceless drug addict but instead tries to get to the root of his problems. Take a look at this passage in which Sonny tries to explain to the narrator why he sometimes felt he had to do drugs:
"It's not so much to play. It's to stand it, to be able to make it at all. On any level." He frowned and smiled: "In order to keep from shaking to pieces." (201)
Baldwin presents us with numerous passages in which Sonny or other characters try to make some sense of his addiction. This helps clue us in to the deep hurt and suffering that contributes to Sonny's problems. The fact that Sonny is also a musician creates even more opportunity for sympathy, since we see that Sonny really has a lot at stake – he has a lot to lose when he's unable to play the piano.
And yet Baldwin is very honest about how ugly drug addiction is. He may be sympathetic to Sonny, but he doesn't romanticize drug use. It's dirty and nasty and painful. It makes Sonny sick of his own smell. And it tears the family apart for many years. Baldwin manages to convey his sympathy without condoning Sonny's decisions. Pretty remarkable for a writer, don't you think?