Many of the characters are so jaded and corrupted by all the madness and sin around them that they can no longer differentiate between good and evil. They think it is pointless to aspire to goodness when it will get them nowhere. They reach a point where they no longer care. They are indifferent to celebration and tragedy alike and do their duties mechanically, without any real sense of purpose. Some begin to lose their ability to feel emotion and become numb.
As they lose their emotional capacity, the enlisted men become more bestial and less human.
Because their apathy negates their fear, it gives the men more courage to fly their missions; thus, apathy could be construed as a positive force.