I was about to go on about alien jailbait and not being Ahsoka, but then I remembered she's a Togruta and this is a Twi'Lek. Long story short I'm a nerd and you can ignore this.
How fitting that immediately after asking her what a Sith would do, he rewards her failure with sex. Because conveniently, Sith don't rape, so she doesn't need to be trained to withstand that. Although why they don't rape, or at least mind break to the point where there would be no resistance to it, is puzzling, considering they always need powerful new recruits to teach. And you would think it's easier to raise them than to have to go looking for them every time. If Ahsoka had vanished after her battle with Darth Vader, and he conveniently had a younger, similarly colored apprentice later on, we wouldn't even need confirmation, it would just be a logical leap.