So I know we’re kinda supposed to feel a bit of schadenfreude over the Frey pie situation, but even if Manderly didn’t technically violate guest right, it’s still super fucked up. Is he due a bit of karmic retribution himself at some point, or does it even out as karmic retribution against the Freys?

According to GRRM by way of Old Nan, no:

“"It was not for murder that the gods cursed him,“ Old Nan said, "nor for serving the Andal king his son in a pie. A man has a right to vengeance. But he slew a guest beneath his roof, and that the gods cannot forgive.”

Wyman Manderly knows the old stories, so I’m absolutely sure that he planned his vengeance accordingly. 

As for karmic retribution, I’m pretty sure he’s planning to die.