Some notes from an on-line commentary:
�Wilde doesn�t say what secret Dorian holds over Campbell. Most likely, it is something that the scientist did years ago while under Dorian�s influence. In any case, Dorian is fully aware that blackmailing Campbell into helping him is dreadful, but he doesn�t hesitate for a moment to do so. In fact, he scolds Campbell for not wanting to help him at first, and he even seems to take pleasure in forcing Campbell to comply eventually. Dorian has become dominated by the evil of his secrets, and he in turn seeks to dominate and control those around him. At this point in the story [when the note is written and Campbell succumbs], Dorian shows that he has surpassed his mentor�Lord Henry�in his power to manipulate.�