It's common for anime to have underdog characters that don't seem particularly skilled as others. Her origin story not only made her motivations more understandable, but made her more likable as a result. While murder is obviously never dismissible, the fact she was motivated by a desire to keep her family safe definitely helped redeem her in the eyes of the audience.

In fact, Rem killing Subaru was an act meant to protect her and her sister from the danger she thought he posed due to him having the Witch's scent. It's discovered that she is deeply loyal and protective of her twin sister, and was tormented and tortured as a child. In the process of saving her, Subaru, as well as the audience, learns more about her personality and origins. The rest of that specific arc ends up focusing entirely on Subaru saving himself from Rem while also saving her. It isn't until Subaru is murdered at her hands that she actually gets development and becomes a main fixture in the story. Subaru has already bonded and fallen for Emilia, and while working at the mansion, Ram befriends him before anyone else, making Rem seem rather irrelevant. When she is introduced at Roswaal's estate, she's actually treated as a relatively minor character. Throughout Re:Zero, Rem is more often than not presented as the character at a disadvantage, whether it is in battle, love, or story relevance. Everyone loves an underdog, and this couldn't be more true when analyzing Rem.

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