They didn't really do either. In the end, I was still just as confused as to why Hei acted like he did, and when he's your main character, you screwed something up. It has something to do with his sister, that's about all I know.
The ending of Darker than Black made me feel like a gigantic idiot because I didn't understand it AT ALL. I felt better when I realized that is because it was a poor ending, not that I'm an idiot (well. I'm still an idiot, but not for not understanding Dtb)