The premise of the show, obviously, is that Walt is in a slow decline and his moral compass has been eroding. But Walt's character is obviously the protagonist, and this sudden shift to being the antagonist strikes me as an arc that will conclude with him coming to Jesus, so to speak.
Normally, it's not all that odd that an antagonist becomes a protagonist. But in this case, there aren't any characters that people want to root for against Walt. People like Jesse, but they don't want to see he and Walt butt heads. Same with Mike prior to yesterday. And I don't think anyone is rooting for the D.E.A. to ultimately catch Walt.
So my thinking is that Walt will conclude this first half of the final season as a bad guy, but they'll bring him full circle in the end. Even if they do that, one wonders what the cost will be.