I think Walt has been pathetic the entire time. He tries to become this Heisenberg character, and it's just that, a character, a form of acting complete with costume change. It's a facade he puts on to appear tougher and more in-control than he really is. Walt's basically been winging it all along and has become more destructive to himself and others over time.
Jesse leaving was hurtful to Walt's pride, his sense of routine, and his whole "empire" build-up, but as we saw this ep, he tried to overcome that very quickly, almost reflexively. He simply used another young guy as someone to mentor over and to act as basically a surrogate Jesse, and continued forward. He doesn't tend to let things stop him for long.
When it comes to Mike's death, Walt will do what he eventually always does when he's the cause of some unfortunate act: he'll rationalize it and move on. With the events surrounding Jesse, Mike, and everyone and everything else, Walt views them as temporary stumbling blocks, and his defense mechanism continuously pops in to fix the problem and he moves on. And now you're starting to see him truly crack as all of the pressure from that builds up; the fidgety, hesitant Walt you saw shoot Mike shows he's starting to crumble to some extent, and that he won't be able to keep the masquerade up for much longer.
Walt can't allow anything to stop his forward momentum in any way because he'll then actually have to consider all of the actions he's taken in the series. The guy went off the rails a long time ago and that train is about to crash hard.
Man, it's amazing how opposite your view of Walt is from mine on this. To me he's always been heisenberg and his 'bumbling/pathetic' side is a mask for much deeper problems with his psyche. He's always been a liar and able to rationalize the most heinous things. It took getting cancer to drive him to do what he really wanted, but I have no doubt he's exactly where he wants to be and he's comfortable with everything he's ever done.
Sometimes he seems shocked or shaky at what he's just done, but there's always this glaze over his eyes as he realizes just how easy it is for him to do things other people find shocking. His face with Mike's gun to his head in the last episode? To me that's the true face of Walter White.
Also if they renew this show I won't watch an episode past its real ending. This show *needs* to end, and anything less than a real ending would be a failing its audience.