As can be seen with both Roswaal and Rem, Ram was very passionate about her love for the pair in wanting to bring Roswaal his underwear and in making sure Rem gets less work and gets validation from others. Rem herself falling so hard for Subaru and having previously done so much to be a stand-in for Ram due to having thought of how it’s great that Ram lost her horn also supports this read of oni/demons feeling great passion for what’s important to them.
As such, since it was revealed in a Break Time segment that Roswaal was no longer allowing Ram to bring him his underwear, after she revealed she loves him, more of her passion was directed to Rem, who shares the same oni blood in her veins as she did and could feel something was amiss in her life as Julius did when he fought Roy Alphard, leading to her reacting angrily to amnesiac Subaru saying that he’d lost his memories since she felt a sense of powerless due to not having memories of her and also ’cause her relationship with Roswaal wasn’t as great as it used to be, which increased the baseline stress she felt.
To Ram’s knowledge, Subaru’s the only one who remembered Rem and was her anchor to her sister, and given Rem’s contributions in Arc 2 and 3 were erased, and everything Rem had had prior to that was forgotten by her, of course Ram would flip out since Subaru was Rem’s greatest champion. To Ram, Subaru forgetting Rem is like having Rem’s greatest hope disappear, which she was personally hurt by as well as feeling hurt in place of Rem since she can’t feel anything right now and given the saying on how you only die when the world forgets you even if I believe that Foxidna remembers Rem since she’s given hints that she remembers Julius.
In a sense, Ram also kind of glazed Subaru hard in that scene since it showed she thought that if there’s one person in the world who would never forget Rem, it was Subaru, and that was part of the reason she reacted as strongly as she did given he had charged at her to attack her, so I believe this makes her reaction more justifiable given she thought that it was a possible imposter instead of the real Subaru.
I’m not saying people can’t dislike or hate Ram, but Tappei intends for his audience to see the perspectives of all the characters in his series, not just Subaru’s. Self-inserting into Subaru to mald against those who’ve hurt Subaru physically just hurts one’s ability to comprehend and enjoy the story as Tappei intended, which is to treat it as a serious work and not like generic male power fantasy isekai series and see how flawed characters would react and struggle much like everyday people do.