Jeeze the most disturbing part of the final scene is when Walt is pacing around like he regrets what he did and then says, "Oh shit I could have just done this to Lydia. Shoot! Darn."
Its like he doesnt even realize what he is doing any more.
I feel like that line is about him getting control over that impulsive part of himself that couldn't stand being told his place. I think there's an internal monologue going on there where Walt is saying "I shot him because I needed the names. I couldn't just let him go" He's rationalizing away his impulsive and angry action.