Why Would Beric Dondarrion Want to "Help" Catalyn?/Spoilers

[baksh]

Well, I think that Berric is an honorable man, well atleast as he and others say it, and he thinks that the way the Freys killed her and Robb was a dishonorable act and they should be answerable for it. That makes me wonder that if indeed the Freys sew Grey Wind's head on Robb's neck, and what if The Red Priest tried to bring Robb back to life in that situation, what would have happened? Scary.