Muraki's loved Tsuzuki for so long. He's chased, teased, and lured the Shinigami at the risk of death, and now it's all come to its climax. And Muraki is so proud, so ecstatic, in the afterglow. The Shinigami is his.
Tsuzuki had been broken for so long, covering it up with a smile. Muraki ripped off that cover, exposing all of the sadness and anger, all of the pain. By breaking the Shinigami once more, Muraki claimed him. He was truly the doll Muraki had always wanted.
Muraki stands watch as Touda's flames lick the roof of his laboratory. His own body was a necessary sacrifice, one easily remedied... Worth it for the glassy-eyed look on the faces of his two favorite dolls after they died embracing each other. Muraki glories in the afterglow for a moment longer, then slips back into the shadows, knowing he'll cherish this memory forever.