Poor Alan Campbell. He must have done something wrong at one point, since Dorian has solid blackmail material… but still, he just seems like a good guy who got caught up in the wrong crowd.
He's an earnest scientist and musician, and, from his description, we gather that he's quite brilliant. However, Dorian had to come along and ruin everything. He took this promising young man, befriended him, presumably corrupted him, and then the two friends broke up— something must have gone awfully wrong. When Dorian summons the wary Alan back, we can tell that he's tried his very hardest to put his friendship with Dorian behind him, but alas—you can't ever escape Dorian Gray, it seems.
In the end, after he's forced to dispose of Basil's body for Dorian (ugh!), we later find out that he commits suicide.