Well to me an addiction is a type of high that a person gets and feels that they need to do or they feel horrible. So I don't see warlocks and black magic like that. I just see them twisting into someone who gets more and more comfortable with doing horrible things.
I don't think it's addictive in a literal, chemical dependency sense. IMO it's a matter of, a person's actions always shape who they are, but magic does it
much more strongly since the connection between the act of will and the result is far more direct.