It's difficult to talk about the drama without reference to the book, so please don't read the following if you don't want the book discussed.
Re the vicar, i think one of the things that let the drama down was the lack of explanation as to why he chose the path he did. It left him looking like a pantomime villain. He talked about the church being a lie and about the power of the pagan gods and that was about it. In the book much is made of his appearance and otherness, which is lacking in the tv version, and it is this that separates him out (he is an albino and is described as having male and female attributes). I think i read that his sister was added to reflect this female dimension (she's not in the book), but i don't think it worked.
He describes a sickness of the soul (or something like that) caused by the modern age. I think in referencing the pagan gods he may have been referring to how in ancient times someone like him would have been seen as something special, rather than something to shun (the theory that shamans and holy men may have been people with disabilities or characteristics that set them apart from the ordinary, making them closer to the spiritual). I imagine that his uniqueness meant his journey to adulthood may not have been particularly easy given the times he was living in, and he sought solace in the church, but found he had no faith in it.
The book doesn't really make clear why he turned to wrecking (other than giving him a sense of power over others) or how he had so much power over Joss and his men, but i would think fear of that which is different may have played a part, the recognition of something that is truly rotten at the core, and for Joss the relationship works well. He probably wouldn't have been able to manage such an enterprise on his own, and his role gives him standing and respect among his community.
It is one of the main themes, how that which appears dark and evil (Joss) is eventually revealed to have a streak of decency (or at least did once), whereas that which appears light and good is the true evil.