With the acid bath, I don't think it was wrong to have it - my only problem with it is the fact the Doctor seems unconcerned or even pleased that two men have just died horribly. That feels wrong.
It was this I think that made the whole thing feel darker. It's the Doctor we rely on to be moral and concerned. He's had to kill or cause the death of many people and things in the past but he's never (as far as I can remember) seemed quite so proud of it before. Even though he didn't cause their deaths this time, his reaction to it I think was particularly dark and unfeeling
