What we know about the transition of power in the 760's is essentially known trough Carolingian sources, without giving Carloman's or his supporters point of view. Truth is, we don't even know what the causes of Carloman's death were, except it was unexpected.

Even these sources point, however, that brothers were at odds and tied alliances to reinforce their situation against each other. It didn't went into a full civil war, however, because of their mother's intercession but maybe as well trough a political and/or ideological sense of need for dynastic unity.

This sense of unity is highlighted by the composition of Charlemagne and Carloman's demesnes : while Carloman's looks as a bloc it is actually made of quite distinct entities (Tassilo of Bavaria managing to escape Frankish suzerainty), and Charlemagne's couldn't be efficiently ruled without dealing with Carloman (hence why Carloman's refusal to support his brother against Aquitains in 769 was an important geopolitical matter).

When Carloman died, his son Peppin was deprived of political inheritance and wasn't made king. But Frankish kingship (as it was customary elsewhere in Europe) had a strong political-military function and while child-king existed in Merovingian Francia, it was generally trough a regency or the predominance of the majordomos : it worked principally because Merovingian dynasty had an almost sacralized function that Carolingians didn't had in a same way and as the latter wanted to appear as a strong dynasty among Frankish aristocracy, it wasn't really an alternative.Eventually, Carloman's own truste was divided on this matter : most accepted Charlemagne's taking over as these aristocrats didn't want to loose access to palatial power and benefit, that Pepin couldn't have them (even if they accepted to be ruled over by a child) , and only part of them fled in Italy.

Would a murder have been really that necessary? The struggle between both sons of Charles Martel, Peppin III and Carloman (I), then the fight against Peppin and his nephew Drogo was quite similar to the enmity between Charlemagne and Carloman : it ended up with both Carloman and Drogo being sent to monasteries, de-legitimizing them for claiming royal authority.There wasn't a real issue for Charlemagne to do the same with his brother, and what he probably did to his nephew eventually.

An assassination, however, would have been consequential : not only risky at best in a context of a "cold civil war", it could have set a lot more of Carloman's supports against Charlemagne, as it would have threatened their position; it would have been at best frowned upon by a church Carolingians tried to ally themselves with (especially as Lombards were both allying with Carloman at this point, and trying to tie influence with Papacy), and eventually caused a serious crisis maybe comparable to the Merovingian faida in the VIth/VIIth centuries.

Did Charlemagne killed Carloman to take over his brother's dominion? It's technically possible, but not only there's no evidence, direct or circumstantial, but it simply might not have been necessary in that matter, or even harmful to his ambitions.

- Before France and Germany, Patrick J. Geary

- Les Carolingiens, Pierre Riché

- Vita Karoli Magni, Eghinard