Because the first few episodes leads you to it. You get this cool main character who takes it upon himself to purge the world of those who can't be made responsible for their evil otherwise, and then you get the detective angle. It clearly tries to be Holmes Vs Moriarty type of thing, except with the opposite PoV. If you didn't get those expectations from the show itself and enjoyed it, good for you! It was a different experience for me, especially with how much praise I've heard about it beforehand