The whole point with Vader is that Anakin's dead until Luke saves him. Why change that? Vader sure didn't spare Luke's arm when he got stabbed in EP5. So i feel this is out of character for Vader to just spare Ashoka. I like her too but... Vader is Vader. He shows no mercy. He should've killed Ashoka. Without hesitation.
The point is that Vader/Anakin is always in some level of conflict (as Luke comments in RotJ) but that the dark side had essentially taken over. Presumably there was some affection towards her, and the Sith are creatures of Emotion. Somewhat ironically the Jedi were taught not to let their emotions cloud their judgement; with the Sith it is encouraged.
So while it might be out of character to be sympathetic but not to be conflicted. He likely would have had much stronger emotions towards Ashoka than he did towards Luke. He developed those feelings for Ashoka while he was still good (possibly the last tiny bit of good left) while his feelings towards Luke formed as part of a political power play initially at least.
He's not called the Dark Lord of the Sith for nothing. Vader wiped out any personal opposition before A New Hope. There is no Ahsoka, Kanan, Ezra, etc, in the Classic Star Wars Trilogy, nor are they mentioned or even remotely referred to.
Early 90's games like Star Wars TIE Fighter and Star Wars Dark Forces do more to compliment and respect the Classic Trilogy than this show ever could.
Can anyone honestly see Disney ending this show in 5 or 6 seasons with the above Rebels all dead though? Not a chance. In fact we'll probably see more bumbling and stumbling Imperial officers and Storm troopers time and time again on the losing end.
Fine by me. The expanded universe of these shows is better than episode 7 where nothing has apparently changed in the last 30 years and most of the main characters from before shirked their responsibilities and ran away.
I want Ahsoka to have been killed by Vader as well. It would really sell the fact that Anakin is dead until ROTJ. Vader needs to feel like a threat and I believe Obi-Wan is his only notable kill of a force-user that we thoroughly saw in the current canon. On top of that, Obi-Wan sacrificed himself. Sure you can say that Vader drove Obi-Wan into a corner, but Obi-Wan let Vader kill him.
"My insecurities could eat me alive" - Marshall Mathers
He should have killed Kanan, Aladdin, and the others as well. Instead he "let them" drop an AT-ST on him. He could have killed all except one and let one escape. He could still have tracked the person by doing that.
His threat level is down to 0 now.
Kenobi said, in the OT, that Vader hunted down and killed the jedi. Uhm...hasn't hunted down a single one yet. He just keeps letting them get away.