I really hesitate to post this, given the way this thread has gone, but there's a simple question that I cannot figure out. Maybe I'm missing something really obvious.
Why kill Lydia?
She was profiting on Heisenberg's blue meth. That's not acceptable for Walt.
Walt retired voluntarily and turned things over to Declan/Todd to produce. He obviously knew she was continuing to profit after he left, she even asked him to come back.
I know, but that was the point of showing the interview with the Schwartzes in the penultimate episode. That interview simultaneously reminded Walt that he turned over his "empire" to other people before (much to his regret)
and it alerted him to the fact that his signature product was still being produced. That's what changed his mind about turning himself in and prodded him to return to NM.
In other words, when Walt gets all mad while watching that interview, it's because he realizes that he's currently making the exact same mistake that he made with Grey Matter all over again. He returns to ABQ because he's determined to not do that. It's sort of a weird moment of clarity.