What in the name of the Old and New Gods did Roose say to the Northern Lords to explain his part in the Red Wedding? He cannot claim he just got away, he has 2000 Frey swords with him, which makes him obviously complicit in the crime even if no one knows it was he who murdered Robb. How can any Northern – highborn or lowborn – justify following a Frey (and Bolton) into battle to themselves?

We don’t know exactly, as far as I can tell. All we know is “Roose Bolton lies about his part in the Red Wedding.” (ADWD) 

But in general, I think his argument goes like this:

1. The Iron Throne has named me Warden of the North, and I hold Winterfell and Arya Stark.
2. We have your kin as our prisoners.
3. I will rid the North of the Ironborn. 
4. Submit and you’ll be treated fairly. Resist and I will destroy you.