Cass is a Soldiers of the One cadet. He is on the transport shuttle with Lacy Rand and Odin Sinclair when it is boarded by a group of Gemenese polytheists who take the recruits hostage. When he is chosen to be the first to die, he freaks out, saying he will do anything to keep his life, even renounce his belief in a singular god.
It is later revealed that this was an STO initial placement exercise. Diego says they needed to find out how the cadets would function under stress - who would crumble and who would step up. No one was actually killed. The deaths were faked and dummies were jettisoned into space.
That evening after the cocktail party, Odin and Lacy observe Cass and a few other students being shot and killed. They were the students who failed the test. Odin tells Lacy never to forget who these people are and what they are capable of doing.