As people bleat about Frey's dual loyalties, are they forgetting about the Lannister soldiers they hung prior to the northmen passing the Twins?

[The Frosted King]

Frey had already killed men representing the king on the Iron Throne.

Those who rail about Robb taking his men and then breaking his oath after he lead them into rebellion need to readjust their timeline.

[Odon]

It's made clear the Freys were defending their territory. Those Lannister soldiers were doing their rape-pillage-burn thing to Frey peasants, so they get treated accordingly. Neither side views this as the Frey's taking a side.