So I saw an insta post about redemption arcs and one of the points was about knowing the limits and that the readers won't forgive a child abuser/murderer so easily,

So now what about a story where, somewhere, a character is forgiven by the rest of the cast, but unbeknownst to everyone, this character has done some horrible shit.

The readers know it, but the characters don't. The readers are left to suffer. :)