Of course there IS the chance Kam just said "They're lying to me anyway" and did his own thing.
But it really was silly for him to say heart attack when a. the signs weren't consistent with that theory and b. well... all the other deaths have been showy (even if COD isn't as such) so why think it was that simple?
One other nitpick though - they really shouldn't have horses riding that close together. Sure, they want to add tension, but I know of some breeds HATING others and you don't want to cause a fight. I personally nearly had a chestnut bite me because he hated ONE Appaloosa (not mine) and thought mine (which was not nearly as grey) was said "mean" horse.