Well, we didn't expect 'Breaking Bad' to end without a little bloodshed, right? It was just a little surprising to me that Mike was the first to go and in such manner.
In a way, it was only logical that the only way Walt could get Mike was to get him by surprise at a moment when he is in an unusually vulnerable moment: persecuted by the DEA, losing all the money he was saving for his granddaughter and stripped of all his arms. In an episode in which Walt had been nothing but an asshole (something he's been doing all season long, really), the ultimate demonstration of Walter White Megalomaniac Unstable Person was his decision to shoot Mike for pointing our how they had it much better when Gus was around.
Mike has a point. With Gus in charge they had a smoothly-running operation, a wonderful place to cook and all the connections necessary to keep it going for as long as they wanted. Hell, they had even get rid of the cartel down south in order to stand at the top of chain. Walt, however, would have probably not be alive at this point had he not killed Gus, so Mike's comment might have made him angry and impotent to a huge degree as he seems to imply Walt is no king of the hill material.
Anyway, next week is the last episode this year and I suppose Walt will have to deal with this pivotal death in some way. I mean, he just killed a man wanted by the DEA in cold blood. This was a very stupid thing to do and might be the first clue in finally guiding Hank towards the real Heisenberg (we now Hank is a capable detective and I believe the show won't end without him putting things together and realizing the true nature of his brother-in-law.)
Another decision that seems stupid in a very similar way to shooting Mike, is teaching Todd how to cook. From what we've seen of this character and the fact that he seems to have kept the tarantula of the little boy he murdered as a trophy, he might not be the kind of person you want to show how to do the one thing that you get leverage from when dealing with other criminals. Walt's inclusion of Todd in the operation is triggered by him being unable to manipulate Jesse into sticking with him after Mike quits.
So, here we have it. Walt has pretty much destroyed his relationship with Skyler, his partnership with Jesse and has now killed Mike. Seems like this batch of episodes will look as the set-up for Walt's downfall once we see the series' final episode.
Also: Is it me, or did Walt leave the frame microphone at Hank's office?