You know even when he's insane, he's sort of a hate to love character
Or love to hate.. whatever you prefer.
But i think that's because of not only Zach's acting abilities, but also because he brings something else to the season. It's not just about 1 thing in this season, it was also about him being the actual killer. Well the original one. And then you find out later he and Lana did have a son and his son ended up being Bloody Face just like Oliver.