Also, I wouldn't play him as having chosen to be a Sith, because that's not something that happens. All Force-sensitive people in the Empire are forced to become Sith, no matter their origins. Nobody ever really gets a choice between being a Sith or a Jedi, it's whoever gets to you first.
The Star Wars universe is a cartoony black hat/white hat universe that is best taken as a starting point for improvisation. When one takes it too seriously, its many and blatant lapses of logic become glaring.
There is no logical reason why a force user trained as Jedi couldn't quit the Jedi and become a Sith, a space pirate, or open a hotdog stand in the fleet. There is no Zeus to smite you for not using his gift of magic appropriately. When people can travel anywhere near instantly, there is limitless potential for storytelling and limitless potential for variety of characters. The George can place whatever restrictions he wants on his characters in his movies, but these are our characters in a universe we create.