DV: "When I left you, I was but the learner. Now I am the master."
OW: "Only a master of evil, Darth."
He is almost mocking him for his bad choices and his high and mighty attitude. It is better that he calls him Darth, and not Vader because of this context. Vader calls himself the 'master', a Dark Lord of the Sith, or simply a Darth. In this case, Obi-Wan is directly attacking his stance as a 'master' aka 'Darth'. You can almost imagine Obi-Wan saying to him after the previous dialogue, "Look how far you have fallen." It is Anakin's delusion that he is some great master, but the only thing he is a master of is evil, and more importantly, hate.
It may not have been designed that way originally, but knowing the full story as we all do now, that is what I think when I watch it. From a story-telling point of view, understood this way it gives a subtle hint to the audience not only of the bad blood bewteen them, but the stress on the name/title also draws attention to the fact that this is not the name/title by which Obi-Wan used to know him.
Just my thoughts.