Okay...time to do what I said I would do.
I don't believe Vader will kill Ahsoka because the fact is that there is still good in him and that has to be proven to be so.
Now, a true act of evil is done willingly and directly by the person who does it. If Vader stands by and allows someone else to kill Ahsoka (such as Palpatine like he was doing to Luke in Return Of The Jedi) he is not willingly and directly doing the act of killing himself.
It reveals the good in him by not doing such a thing.
In general a being that is 100% good would do exactly the same thing because such a being wouldn't be able to act violently even to save someone because violence goes against the nature of good.
Of course that is where the double edge sword of the matter comes in because doing nothing in a situation like that can be considered an act of evil as well.
In other words good taken to its extreme natural nature can cause one to do something evil.
It would be Vader's bond with Ahsoka that would cause him to not kill her himself revealing the good that is still in him, but still showing his evil by doing nothing to save her because his bond with Ahsoka isn't strong enough to overcome the dark side in order to do the act of saving her, unlike the case when it came to Luke.
[This message has been edited by Shifter (edited 07-24-2015 @ 04:23 PM).]