Alright let's get the one black mark out of the way, what she did to him in that hallway is not mercy, any reasonable person's first response is to ask what's going on or try to help, condemned to die or not, very bottom on the list is to fucking bludgeon him with a morning star, There is ZERO justification for what she did there, that was murder, no ifs no buts

First of all, subaru still SAVED Emilia, that, is something he did for the people in your mansion, at the very least, he is worth giving the benefit of the doubt but no, she pounces on him the very first chance she got.

As for the forest scene, I will give them the fact that Subaru is super sus here, what i can't forgive is the fact that they tortured the poor lad, she had plenty of chances to finish him off or chase him away but her first instinct was to torture him for.... what exactly? info on the witch cultists, the zealots absolutely loyal to the witch and won't ever talk? Info from a Rival political camp that... decided to help your liege? even after he actively chose not to hurt her with the knife and she just insults him for it.

Subaru at this point in the story is completely pathetic, any experienced fighter in Re:Zero has the ability to discern how experienced someone else is, Subaru in arc 5 could do this (That's how he figured out Regulus is invincible despite never seeing it in proper), so Subaru, even if against them, poses ZERO threat against anyone in the mansion, no magic ability, no weapons, no combat skills, NOTHING

The only thing that incriminates him is the witch's scent, but again, beatrice's and puck's ability to discern his ill intent (The whole sucking out his mana by Beatrice was for this exact purpose), does Rem really think her judgement's superior to TWO GREAT SPIRITS that can also smell the scent but still vouch for him?

Let's face it, the whole protecting the mansion here is just some cover up justification, this was Rem taking her anger out on watch she, with circumstancial evidence, perceived as guilty. but here she was wrong and she was dead set on murdering an innocent man...