- It was part of Roose’s conditions for agreeing to go along with the Red Wedding. He knew he’d need military help to subdue the North.
- Roose’s married to Fat Walda, his son with her will be Lord of Winterfell and Warden of the North one day, which makes the North the richest prize the Freys got for the Red Wedding. They want to protect their property to be.
Roose’s son on Walda will be heir to precisely nothing unless Ramsay dies, yes? Being as how Ramsay is the older son who has just been legitimated? It is Ramsay who will inherit the Dreadfort, Winterfell, and the Wardenship. (In theory.) The Freys don’t get a taste of that pie.
And even if Ramsay died, Roose and Walda’s hypothetical kids would not be the heirs to Winterfell; that line would go through Ramsay and “Arya’s” children.
I’m pretty damn sure the Freys were intending to kill Ramsay so that Walda’s kid would inherit. Not that they would have succeeded, but that was the plan.