Because it’s not Catelyn anymore, it’s Lady Stoneheart. She’s a revenant, and she’s fighting for vengeance not with a scalpel, but with an axe. Catelyn wouldn’t have hanged Brienne either, but it’s Stoneheart.
Thanks for the question, Anon.
SomethingLikeALawyer, Hand of the King
Indeed. And specifically, Stoneheart’s got a spy in Riverrun (Tom o’ Sevens), which means she may well know about Sybell’s deal with Tywin. If so, Stoneheart will probably assume that Jeyne was in on it.
Or just blame her for Robb’s death because of her marriage to Robb regardless of how little sense that makes.
As SLAL pointed out, “it’s not Catelyn anymore, it’s Lady Stoneheart.” And what Lady Stoneheart is is Nemesis, and Nemesis is the primordial, implacable spirit of resentment against perceived injustice. Nothing rational or fair about it.