I miss out on this thread. I'm a huge big Breaking Bad fan.
Here's my theory. Walt tried to rationalize the whole situation to himself and others by with the 9 names, but deep down he wanted to kill Mike regardless due to his ego and anger over Mike having no respect for him. You look at this whole season, and Mike was the only character that didn't fall for any of Walt's sh*t. In every other character's case, Walt has either been able to strike fear in them (Saul, Skyler, Lydia), manipulate them (Jesse, Hank, Marie, even Declan), or been loved or respected by them (Walt Jr., Todd). Mike didn't have any of that for him. He still looked at Walt like the weak p*ssy ass chemistry teacher even after he killed Gus and that really bothered Walt.
Walt wanted to be recognized as the next Gus Fring, and for whatever reason getting Mike's validation would achieve that, but Mike told him multiple times throughout the season that he wasn't Gus Fring. And you look at this episode, and throughout the episode, Walt is looking for Mike's approval. First, at their headquarters he's expecting a more warm goodbye and some gratitude from Mike, but Mike just brushes him off, then he looks through the blinds and sees Mike shaking Jesse's hand and he's looking on almost like he's jealous. Then in that final scene, he yells "YOU'RE WELCOME" at him.
I also think he was still fearful of Mike, and Mike may have been the only person Walt was still fearful of, and that may have been another reason why he killed him. You look at that whole scene, if Walt getting the names was his primary objective, he would've threatened Mike right away. He would've given Mike a chance to change his mind about giving the names with a gun pointing at him. Thats what cold calculating criminals would usually do if they're doing something just for business and not out of emotion. Instead, he just lets Mike walk right up to him and snatch the bag out of his hand, barely putting up any fight. I suspect part of the reason for that was cause he's scared of Mike and didn't want to risk getting into a shootout with him since he's alot more skilled in that aspect and/or the 9 names wasn't the main issue. It explains the indecisive coward move he did by initially going back to his car then blindsiding Mike at the end when he shot him.
I think his remorse maybe due to the fact that he just realized that this was the first death on his watch that was completely unnecessary. As brutal as every other death was, including the kid's death, they were all done to protect himself or someone he cared about, or protect the business. I don't buy that Walt is completely bad and will ever be. There probably is still a little bit of a human side to him, even if it is very little.