Though Eldon had been considering the likelihood of death ever since he enlisted, he didn’t imagine a scenario like this. He’s clearly acting out of instinct rather than malice, as he’s responding to the realities of their desperate situation on the battlefield. In the ultimate example of the friends’ ability to communicate, Jimmy also clearly assents to what Eldon decides he must do. It’s also an extremely traumatizing event, as Eldon ends the life of the only friend who’s ever made him feel at home in the world.