My impression is that something "snapped" inside Walt since the end of last season. He doesn't care anymore. After all, he told Skyler, "you don't have to think about all that stuff. If you just let it go everything is going to feel fine." (Well, something like that anyway.) He's even trying to convince Jesse (and himself) that he cares about the dead kid, but afterwards he's just whistling happily while doing his thing.
But sure, I can imagine all that stuff will start rising back to the surface at the end when he inevitably gets caught/is going to die. But then it would also be a bit interesting if it ended with him saying something like, "I regret nothing."