Start of Darkness is lengthier than On the Origin of PCs, not just due to its page count, but also because he only focuses on two characters instead of six. In addition, while Burlew has already taken pains to give us the back stories of the protagonists, the antagonists appear to be evil for the sake of being evil. Here, he gives us an understanding of why they're the way they are. Xykon has never been a complex character (aside from his ability to stay one step ahead of the heroes and come up with nasty punishments), so his background reflects that, but Redcloak's complexities make him an interesting character, even if we don't like him that much. Here, Burlew gives us why he's a complex character, and gives us more insight into his motivations.