The Dinner Party

You walk in to the large bedroom. There is a desk in the corner of the room. You walk over to it. You see a pile of paper and start to rifle through them. It appeared to mostly be bills, but you see one red envelope with a large purple seal with the letter M on it. You look to see if anyone was watching and quickly open the letter. It was a letter from someone named McCallister. It was describing some sort of plan to murder people. Now you knew that McCallister was the killer but could the host be in on it too?