Agamemnon managed to turn all the kings against Achilles simply by pointing out his pride—apparently the other kings also refuse to see themselves as beneath Achilles. Meanwhile, Patroclus had assumed that just because
he would find it morally reprehensible to allow Agamemnon to take Briseis, Achilles would, too. Both he and Patroclus know that Agamemnon plans to sexually assault Briseis, but Achilles is willing to let it happen in service to some plan. Put another way: Patroclus’s primary concern is Briseis as a person; Achilles primary concern is his honor and conflict with Agamemnon, and he, like Agamemnon, us willing to use Briseis as a pawn in that struggle.