McNulty's "spiral" feels sudden because it makes you wonder: what was the point of domesticating him in the first place if he's just going to revert to his old ways without giving weight to the consequences? The fact is, Dominic West wasn't available for much of last season, so they had to find some way to explain his absence. It's becoming clear that the writers were never really interested in his sober storyline, and what they did have of him last season was really just filler.