Throughout the whole Obi Wan series, we saw how obsessed Vader was to find Obi Wan Kenobi. This determination stemmed from Vader's defeat on Mustafar, where his 3/4 of his limbs were severed and he was left to burn and die. A large part of this perseverance in Vader probably also comes from his belief that Obi Wan was holding Anakin back from his full potential.

In Tales of the Jedi, Ahsoka Tano, Anakin's apprentice, is revealed to have encountered and defeated an inquisitor prior to the events of Star Wars Rebels. Unlike the feelings Vader held for Obi Wan, Anakin didn't hold any hostility (to my knowledge) towards Ahsoka. He might have had some jealousy that she had the strength to leave the Order and that he didn't, but otherwise, they were practically siblings. 

In Rebels, we see that the cold and collected Vader is in pursuit of the rebel fleet (and Ahsoka), but he did not display the same obsession for Ahsoka that he had for Obi Wan. Part of me has to wonder if, in Rebels, Vader truly wanted to kill Ahsoka the same way that he wanted to kill Obi Wan and other Jedi. Part of me also has to ask if Vader would have been more of his monstrous, self-destructive person had the inquisitor in Tales of the Jedi survived and reported Ahsoka's survival to Vader. Tales of the Jedi was set far earlier in the timeline than Rebels and the Obi Wan series. Vader was still in a phase when he was rampaging as a Sith, rather than being the calm and cool Vader that we all know and love.

What are your thoughts?