Correct me if I'm wrong, but this is the first time walt killed someone not out of necessity
This occurred to me as well. There's no "them or us" rationalising, no acting in self-defense or trying to save his own skin to hide behind. This was cold blooded murder, at best done in the heat of the moment out of anger, and at worst at least subconsciously pre-meditated, with Mike's fate pretty much sealed in Walt's eyes as far back as his "throat cut" monologue that ended episode 3. He killed somebody when he knew full well that no good would come out of it, for no reason other than to make himself feel big. Gus would never have done that.
Of course, Walt has been utterly vile this whole season, with little in the way of redeeming features, and you could argue he wasn't much better last season. But maybe it took the offing of a beloved viewer favourite like Mike for some people's rose-tinted "Heisenberg is totes badass" specs to finally be shattered.
I'll be sad to see Mike go. From bit-part player in Season 2, to intriguing recurring guest star in Season 3, to fascinating supporting character in Season 4, to integral part of the fabric of the show in Season 5, it's been amazing how this character has grown and how excellently Jonathan Banks has portrayed him throughout. In his warped way, he was just about the most decent, principled person in this murky world. And I think his death declares with a tone of finality that the era of decency and fair business that Gus tried to maintain (as much as one can in the meth business) has well and truly come to an end. We're in the Heisenberg glory train now, and the titanic ego driving it is going to send it crashing off the rails fast. I'm sure it'll be excellent, horrifying viewing.