They thought that Sabet and Hoori are bluffing when they attacked Matt. I guess they were hunting for the D and they came to the conclusion that Matt is the D. Then Hoori and Sab attacked Matt.
Fred thought Sab and Hoori are acting just so Matt won't be whacked. They thought Sabet the lawyer will save Matt from being lynched. That's why they whacked him.
It turned out really bad, but it had some logic in it.