This is great, but the line in the movie always annoyed me.
Darth is just a title, so it doesn't make sense to call him that. He should really say "Only a master of evil, Vader". Otherwise every sith would just be called Darth.
Yes, I figure this was due to a bit of retconning on Lucas’ part. When he wrote Star Wars: A New Hope, perhaps he intended “Darth” as a name rather than a title. It wasn’t until the Phantom Menace he introduced the usage of Darth as a title.
Even so, how many times are other character addressed by their titles, such as Padawan, Master, Senator, etc., not to mention military titles like captain and admiral. In that context, I think it could make sense for Obi-wan to call Vader “Darth.”
I think there's enough room for interpretation for Darth to make sense. You could even postulate that addressing him purely by his title and without his name is Obi-Wan putting a spotlight on the fact that Vader has become less a person and more his position.
Yeah, it's a bit of a retcon, but one that was handled well, albeit clumsily. Darth was probably going to be his real name at one point before they decided it was more appropriate as a title.
I always like to re-interpret that scene as Obi-Wan using a bit of wit in the heat of battle to chastise his former apprentice. Him referring to Vader as simply "Darth" is a refutation of Vader's claim that he is the master/more powerful one, and it drives his point home that he would never be better or more powerful than a Jedi Master because he had already embraced the Sith ideology and even taken their title.