When Vader fought Obi-Wan Kenobi, and had his helmet opened on the left side (unlike Ahsoka vs Vader who opened it on the right), we see Anakin's conflict with the dark side as his eyes shift from Blue to Sith yellow / red.
I feel this is a perfect portrayal of Vader's conflict within the darkside itself, even as Sidious / Palpatine explains it perfectly to him on Mustafar: "Perhaps your feelings for your old master have you weakened".
I think this is exactly what is happening where Anakin can't fully commit to the dark side when he's in front of Kenobi.
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