War between the brothers was dawning and Carloman was attempting to team up with the Vatican. The sudden death in his young age (20 ish) established Charlemagne's rise to power. The common consensus seems to be that he died a natural cause. Why is that? Not to forget that their mother, Bertrada, obviously favored Charlemagne and schemed against Carloman before. Its just all so incredibly convenient for Charlemagne.

He prooved to be ruthless at a younger age, on the other hand he seemed to very religious and petricide is a major sacrilege.