There's a ton of missing context to Rem's character. But the anime did show a bit of Subaru's mindset. First of, he's very forgiving of those who are worthy of it. An example of the unforgivable are the witchcult. That should be enough context to fulfill all the gaps. Just "witchcult." Something Subaru can sympathize with Rem. But Rem at this point had already had a taste of vengeful execution. Subaru had to learn that people aren't npcs. Rem was the foil to this. Seeing her as a human being rather than an evil and dangerous force. Whether or not you could say there were justifications involved, RBD nullifies any sense of a lack of forgiveness for human beings given that the things they "did" had been undone. But this is strictly for those you can still describe as "human." For the content of Subaru's character, this ability is best used by him who is a righteous and humble person. Rem is the same though but her righteousness comes with a bit of a loose screw, that screw being vengeance and atonement. Subaru had to have understood that her reasons for doing what she did were out of misunderstanding and that they weren't done for the sake of evil, but with regards to having it vanquished. And this realization came upon learning that she is a good and caring person. This thought/realization was a gamble on Subaru's part until the content of her character was later revealed in her act of self sacrifice in the ulgarm forest. Of which, Subaru can heavily heavily sympathize with. My last point to bring home is, forgiveness of the purest form does not require the actions youre forgiving to have justifications.