Sith/jedi
Firstly, If the emperor knew of the prophecy of the chosen one would balence the force, he must of realised that anakin would betray him, which suggests that his prophecy was along completly different lines.
Also, if anakin was a jedi and then became a sith, does that mean that any jedi can become a sith if beconed by the dark side. And could potentially a sith become a jedi. What is a sith anyway is it the jedi equivalent of the dark side?
Ok..
From what I understand from what GL has said, no-one s CERTAIN exactly what the Chosen One in the Prophecy is meant to do. Palpatine probably thought that he had neutralised any possible threat from Anakin when he turned.
Second- any Jedi can fall to the Dark Side but it is apparently very rare, presumably due to their excellent training.
Becoming a Sith, we would guess, is more difficult, because there are only two of them! So a Master without an apprentice would have to find you and train you as a Sith.
The Sith are an ancient organisation thayt was totally dedicated to the Dark Side (in the same way the Jedi study the Light). They used to be a serious problem but all of them being power hungry maniacs they fought each other as much as they fought the Jedi and the Republic, and were eventually decisively defeated.
After this defeat they started this 'Only two' thing, to help avoid detection and because, quite frankly, any more and they would start fighting each other again.