Kind of a frustrating finale. It didn't give us a straight mystery -- there was no way to deduce in advance who the real killer would turn out to be. It was just sprung on us out of the blue. This show these days is overly fond of having big developments happen off-camera and be revealed in dialogue after the fact.
I was really hoping the Michael plot was building up to the reveal of Moriarty as the mastermind behind him. When it turned out the killer was a woman, I was hoping that's where it was going, maybe to set up a Moriarty arc for next season. But the way things ended, it feels very much like the producers expected this to be the series finale. I wouldn't be surprised if they always wanted to end the series with that scene, but it felt like they rushed and forced the situation to get it to that point within the last couple of acts.
Of course, CBS has already renewed the show for another season, so they're going to have to figure out how to get out of that situation and somehow restore the status quo. Although I wonder if they'll actually dare to change something permanently, like actually replacing Marcus -- or maybe replacing the captain, if the truth of what happened came out. Heck, I think it'd be cool if they just stayed in London.