Small and slight, Thierry is the Timon to Pinabel's Pumba—if those furry cartoons had been fighting to the death over Simba's innocence.
Thierry's reasons for supporting the death penalty are also complicated and equally interesting as to what they say about Frankish society. He grants that Roland may have done something wrong, but argues that "the fact that he was serving [Charlemagne] was sufficient to safeguard him" (277.3828).
In fact, he and Pinabel are coming from the same direction, both arguing that social position should influence someone's trial outcome. For Thierry, Ganelon's attack on one of Charlemagne's most beloved and trusted knights was the same as an attack on Charlemagne's entire feudal system of government.
Thus, even though Ganelon claims that Roncevaux was a matter of private revenge, Roland's high social position and military value make it a matter of public treason. God clearly thinks so too, since he intervenes in the trial by combat—totally unfair—and gives Thierry the victory.