Subaru returned by death in front of the door.
"Huh, didn't expect him to kill me." were his thoughts
Maybe in the past Subaru would have been shocked or angry, but not now. He had used Return By Death so many times that he no longer cared, he did not value his life at all.
[Natsuki Subaru] would have probably valued his life more, but [Natsuki Subaru] and Natsuki Subaru were different people. [Natsuki Subaru] was a great hero who had no need to use Return By Death. While Natsuki Subaru was the fool who was only good at dying over and over again, it was the only thing he could do.
"I will never be like [Natsuki Subaru], I can never be that good." Those were Subaru's thoughts
There was once a time where Subaru had foolishly believed that [Natsuki Subaru] was the same person as him, back in the Pleiades Watchtower, but now he knew that was not true.
Rem and Emilia would have never done that with Roswaal if he was [Natsuki Subaru]. He was only an inferior version. They realized that and so they moved on to Roswaal. If he was more like [Natsuki Subaru] then they would still be with him.
Subaru wondered, if Emilia and Rem were able to figure out that he was not [Natsuki Subaru] then how long would it take until everyone else found out?
Garfiel, Otto, Beatrice, Roswaal, Ram, Frederica, Petra, Meili, what would they do when they discovered who he really was?
Subaru knew the answer, they would shun him, they would do the same thing that Rem and Emilia did.
He knew that he deserved it, he deserved their hatred, he had failed them so many times. He had killed them with his uselessness so many times until [Natsuki Subaru] showed up to save everyone. Then Subaru would steal the credit for the work that [Natsuki Subaru] did and steal the love meant for [Natsuki Subaru].
He knew it was wrong, he knew that he didn't deserve it, but Subaru was a greedy pig. He wanted it all, he wanted all of the love they had for [Natsuki Subaru]
But now he was scared, after seeing Rem and Emilia do that with Roswaal, he was scared of what everyone else would do if the knew the truth.
"I can't let them find out."
But how? How could he stop them from finding out? Emilia and Rem already knew, and he couldn't return back far enough. What could he do? He could never be as great as [Natsuki Subaru]. They would see the differences eventually, they would see the flaws that [Natsuki Subaru] didn't have.
Subaru knew that he could never be like the great [Natsuki Subaru], but perhaps he could mimic him. Subaru remembered back when he had gone to the Pleiades Watchtower, he had a plan to become [Natsuki Subaru] by reading the books of the dead of everyone. It was a flawed plan, he could never truly become [Natsuki Subaru].
However he could learn more [Natsuki Subaru] and try to mimic him, if anyone noticed that he wasn't [Natsuki Subaru] he could just use Return By Death in order to stop them from finding out.
If he did it for long enough, eventually even Emilia and Rem would believe that he was [Natsuki Subaru] and he could have their love once again.
It was a perfect plan, and now Subaru knew what he had to do.