With Tate/Hastings being unveiled as the killer, a lot of the mystery is gone, but it's been replaced by the suspense of what will happen to Gus, and by Marco in a fine performance of a cop who's trying to find and save his own son.
Exactly what Tate wants actually still is a mystery. Sonya correctly says he won't kill Gus because he wants to prolong the agony Tate is putting Marco through, but Tate is sure playing it close with Gus, just as he did with Alma and the girls. Which raises the same questions: Is Tate's goal to have Marco see the death of his children, or narrowly save them - and, if the latter, why? Possible Marco is so quietly deranged that there's no discernible logic in what he's doing, but that doesn't seem all that likely, either.
Tate manages to get Frye from the frying pan of an alcoholics confession group into the fire of who knows - in a very effective scene - so Tate is at least being consistent in meting out punishment to everyone who helped destroy his family. But, again, to what ultimate end?
Presumably Charlotte had nothing to do with any of Tate's woes, but you never know, and, in any case, she's become quite the killer herself. Indeed, her evolution from clueless, wide-eyed widow to tough don't-mess-with-me-or-mine lady is one of the more important transformations on the show. I'm hoping she survives this season, because she'd be great to see in the second season, which I'm also hoping comes to be.
See also The Bridge Opens Brooding and Valent ... The Bridge 1.2: A Tale of Two Beds ... The Bridge 1.6: Revelations ... The Bridge 1.7: A Killer and a Reluctant Professor ... The Bridge 1.8: Some Dark Poetic Justice ... The Bridge 1.9: Trade-Off