In this episode we learn more of the past of L, Beyond Birthday, and Mello and their lives at Wammy's House.  L tells Misora that she knows who the killer is. ("The killer..." L said, " is B.")

B is a man who was trained and raised to be a copy of L, a backup if L happens to die. B was not happy about that and deemed that if he is to surpass L, then he would have to kill him.  This whole murder case is just a war between detectives.