Ahsoka hears that the new Sith Lord, Darth Vader, is an ex Jedi though she still cannot comprehend what would want to make a Jedi turn to the Sith.
Greed, she supposes, lust for power, hatred.
She thinks back to Dooku and when she compares him to Vader she thinks that Dooku seems almost tame in comparison. They say Vader murdered all the younglings in the Temple, they say he turned against his own brother, they say he never listens to even the most desperate pleas for mercy, they say he is not really alive, they say darkness swirls around him. When Ahsoka casts her mind back she cannot think of any Jedi who is or who would do such thing.
One thing she knows for certain is that it is most definitely not Anakin.
Thats what she tells herself anyway.