What has been the evolution or progression of traitor/betrayal/defector games?
These days Mafia is renowned as the worst game in the mafia genre.  The unifying element behind all of these is the hidden information about player allegiance.  It was an interesting mechanic in mafia, but the issue with mafia was that it had player elimination, so players spent a statistical third of the game sitting around doing nothing.  Another issue with mafia is that it was really only good with large numbers of players.  Board game designers being who they are adapted the hidden team mechanic to create better behaved versions of mafia, or include mafia like elements in various games.

Games like Resistance, Avalon, One Night Werewolf, Two Rooms and a Boom, etc. are all trying to address the fundamental problem that mafia had a great concept with a rather poor implementation of it.  (Werewolf proper is just a mafia reskin.)

Games like Battlestar the Boardgame and Shadows over Camelot are trying to include this in a set of broader mechanics to have the game have more intrigue than just a fully cooperative game (such as Pandemic) or a many vs. one game (such as Descent).