I read through starwars.com after every new rebels episode and thought one mention from one of the writers on the site was worth mentioning.
http://www.starwars.com/news/ranking-rebels-10-highlights-from-shroud-of-darkness
Number 9, the episode was really good. But I thought it was a bit odd that out of the three in the temple, Yoda chose to speak directly to Ezra and not Ahsoka whom he actually knows and...in the OT yoda and Obi-wan seemed to be hell bent on preparing Luke to take down his father without the idea of trying to reason with him.
Ahsoka at the end of the episode feels that she has another chance to redeem Anikan. Hmm I may have answered my own confusion.
Maybe Yoda and Obi did want to redeem Vader at this point of time and Yoda thought if anyone could it would be Ahsoka...but since she fails one way or another it makes sense that Yoda and Obi would not want to send another apprentice on a suicidal peace mission.
So that would make sense why in the OT they want Luke to kill Vader, because they think if he killed his own apprentice (who was like a daughter/little sister to him) then he would probably kill his estranged son or corrupt him.