Well it is not the first time, in the web novel too we have seen that Rem tried to kill Sabaru too during his second life. In the web novel there is a very short chapter about Emilia who is carrying Saburu because he is unconscious to the mansion. So, during that Rem notice something that there is witch scent which is coming from Subaru and then Rem wait for everyone to leave the place. After everyone went to their place then she gets close to the unconscious Subaru and thinks by herself thy Subaru is very cute. She thought that it will be totally a waste if she kills him.
So, after that she begins to strangle Subaru and tries to fake it as an accident as she had to confront Emilia and Rosawal too so in order to avoid them she tries to make it look like an accident.
But, at the end after trying strangling him she just leaves Subaru as he is very cute and just flick her nose with her finger. After that she decides to keep an eye on Subaru. The thing is that Rem's family died in the hand of witch cult as well as they massacre the people living in the village too which was very horrific for Rem.
So, the witch cult people are those people which we can call total Psychopath as they enjoys torturing and killing people as well as they don't mercy even babies too. So, they are complete outrageous. Which is the main reason after Rem's family members and villagers died that she holds grudge against them.
So, she wants to kill every last one if the witch cult and whenever she founds someone who have witch's scent she kills them exception being Subaru. But, Subaru smells so much like the witch cult that he resembles them. And the smell that he is carrying resembles him that of a high ranking member. So, this is a reason that he is a threat to the people in the mansion. At first Rem tried her best to kill Subaru at any cost in the mansion loop until Subaru proved himself to be very harmless as well as he care for the people living in the mansion too. So, that was the moment when Rem stopped to kill Subaru.
During the fourth loop Subaru was actually on the verge of death. Also given the circumstance of the curse there was really nothing which Rem could have done in order to save him. So, as Rem was not able to do anything for Subaru, Rem decided to give mercy as death to him. So, these were all the reasons which lead Rem in killing of Subaru.
Which character would you choose, Rem or Emilia in Re: Zero?
No doubt it will be Rem between Emilia and Rem if I have to choose one. The reason is she is way cuter than Emilia honestly. The other main thing for rooting for Rem being the reason is that she is very loyal and supportive and she can give her life or risk her life for the one she truly cares for. I know in one loop she killed Subaru but it was also for the reason that she believed Subaru is threat towards Rosewell Mansion.
Well apart from that in other loop she was the person who always supported Subaru in his journey. Well, Sabaru did his utmost in order to stop slandering of Emilia's lineage by whatever means. So, that was the reason he had to fight Julius. It was very heartless reaction from the Emilia's side. So, after that Subaru fell into deep despair and there was only one person left to support him which was Rem. After that she assists Subaru in fighting the White Whale.
During the very first loops when Subaru was just repeating loops and was not able to save anyone during that time also he turned to only one person none other than Rem. At one moment Subaru even tried to ran away with Rem and marry Rem.
This shows that Rem was always supportive of Subaru unlike Emilia who pushed him away. Even when Rem got killed Subaru tried to save Rem by committing suicide to rewind the time. But, his save point got updated and he was not able to save Rem which was very painful to watch honestly. Now, as I have talked about both Rem and Emilia, Rem is the best out of two.
Why does Subaru choose Emilia over Rem in the very end?
It is a speculation that Subaru doesn't really love Emilia. He had expressed his feelings for Emilia but this were not his really feelings and when he confessed his love to Rem he felt heartbreak. But Subaru even after heartbreak just smiles back accepting his fate. It is a very well written story as the writer simply designed the characters in such a way that even if the characters are right or wrong at certain moments we can just love for who they are.
We have to live with this fact and Subaru have to end up with Emilia. Because, every anime endings can't be sweet forever. Some people are not meant for each other. We see how Subaru suffers as he cries alone at night by walking alone. Another reason being that Subaru mets Emilia first and he had more feelings about Emilia than Rem. Also, another reason being the fact that they were always together on an adventure from the starting so they have feelings for each other.This can also be a reason that Emilia ends up with Subaru rather tham Rem and also Rem was the better choice. Somethings can't be changed and should be left unchanged as it is.