Because...well, because thats what Sith are capable of. But Gev, for all my problems with her, she is correct. Anakin led troops into the temple and slaughtered anyone within. Anyone.
Ask Jacen. I believe he used one of his many skills to witness the event.
Or are you still beyond beliveing that one can return from such darkness to become a force for good once more. Much as I never saw eye to eye with the man in his time here. He too found his way back from the Dark Side. Or from his choices to use the Force that way if you believe in the Potenium.
And Anakin Skywalker didn't suffer HALF what Jacen had to endure