Campbell
Love how in the first episode he's set up as a decent, albeit tough man, and by the end of season 2 he's an absolute monster. Now that's how to flip a character completely 180 degrees.
PS. As a man from Northern Ireland, I must say I was very impressed by Sam Neill's attempt at the accent. A little dodgy at times, but in all honestly, the Ulster accent is very tricky to "do complete right" unless you're a native.