It'd be a first for me, Mice are constant gnawers, if they don't have things to enable them to do so like many other animals their teeth may get to large & kill them. Rats will pick the bodies apart & can get aggressive but it's highly unlikely. It may have been wounded & infection set it, bad food or grain, poisoned in some way. But the mouse would be on the losing end fighting an adult chicken
I would say possible but not likely. If the mouse carried any diseases or if the wound got infected. Not all mice have rabies but it's always a possibility that any wild animal could have or carry it. Unusually aggressive behavior is a sign of it and it would be very unusual for a mouse to attack a rooster because roosters are much larger and are often aggressive. I guess if the mouse was cornered it might fight back but I don't see how that would happen.