why the girl killed Nick (I'm guessing the Vulture he killed was related to her/caring for her)
The guy Nick killed is the same guy that has been taking care of her this whole time and training her this whole time. I don't think that's one of the mysteries.
As to expectations- the high expectations for TWD are born of the fact that it used to meet those expectations, so I think that's fair.
For FTWD, which like you I'm really liking, I don't think it's about unmet expectations as much as it's about either not liking the story-telling technique (non-linear) or hating that we dropped characters and story lines that we left in season three. But I'm with you on FTWD, it's quite good right now.