I'm firmly of the opinion that Bates killed his wife. Not that the show means us to think that, but that the entire thing makes a lot more sense if he's an amazing manipulator who has them all fooled. The actual solution to the thing made no sense to me at all in terms of motive.
I have a love/hate relationship with Downton. I think Julian Fellowes is a writer with a genuine interest in what life was like for all different kinds of people - not just as stock character, but as personalities. And a refreshing belief that most people are basically decent and want to do right by each other. He's also managed to put together a stellar cast who bring a lot more depth to their characters than the script alone has to offer.
On the other hand, he's also a classist old Tory with an obnoxious dewy-eyed reverence for the good-old days when everyone knew their place, and he's been given a massive budget and primetime slot for his apologism. The revisionist history DA displays whenever it tries to tackle any of the actual substance of the class issue or the empire is terrible.
I love the characters, but I frequently end up hate-watching the show. (At times I've caught myself rooting for Branson to torch the abbey.)