Return by Death does not rewind/reset time but actually works kind of similar to the World Lines system in Steins; Gate. When Subaru dies he is taken to a slightly earlier point in a different world line/timeline. Meaning that every time Subaru dies, in that timeline he is dead and the rest of that world lives on past that. For example in one world/timeline, Rem is dead and Beatrice and Ram witness Subaru make an epic speech about doing something only he can do to save Rem and then jumping off a cliff and killing himself. I feel like if this is the case it would be interesting to get a look into one of those alternate timelines, Ram and Beatrice go back to the mansion utterly confused and and need to explain that Subaru went crazy and then ran off a cliff and Ram investigates to punish the killers.