Why does Brinker insist on a trial about Finny's fall in A Separate Peace?
Brinker Hadley is the antithesis of Phineas, law-abiding and conservative in his views. When Finny leaves school for a while, Brinker moves into the position of leadership, but more as an authoritative figure. Disillusioned with the idea of war as glorious after learning some things and witnessing the breakdown of Leper, Brinker desires the truth about other matters. He knows that Leper was a witness on the night of Finny's injury, and when he learns that Leper is on campus, Brinker has one of the boys fetch him.
But, before Leper arrives, Brinker clearly suspects Gene of foul play against Finny. Cautiously, he asks,
"...it wouldn't do you any harm, you know, if everything about Finny's accident was cleared up and forgotten."
Later, "Brinker and three cohorts" came charging into Gene and Finny's room in order to take them to the Assembly Room where Brinker speaks of "inquiry" and "blame on the responsible person." Finally, Brinker answers Gene's question of why they are there: "Investigating Finny's accident!"
"After all," Brinker continued, "there is a war on. Here's one soldier our side has already lost. We've got to find out what happened."
Gene becomes shaky, saying he does not think it a good idea, but Brinker cuts him off, telling him it is being done for Finny's good and Gene's as well. Clearly, he wants Finny to know that Gene's act on the tree bough was one of treachery. Clearly, Brinker is suspicious of Gene, and Leper's testimony indicates that the two friends were on the bough at the same time "like pistons" and when one moved down, the other moved up. But, Leper refuses to implicate anyone. Finny, however, knows; startled by the truth, he hurries from the room and falls clumsily down the marble stairs.