The point was, the grace of it was, that it had nothing to do with sports. For I wanted no more of sports. They were barred from me, as though when Dr. Stanpole said, "Sports are finished" he had been speaking of me. I didn't trust myself in them, and I didn't trust anyone else. It was as though football players were really bent on crushing the life out of each other, as though boxers were in combat to the death, as though even a tennis ball might turn into a bullet (6.93).
What Gene has done by (allegedly) causing Finny's accident is to break the barrier between the war and the innocence of youth. That's why sports are over – there's no such thing anymore, in Gene's mind, as harmless play. Compare this passage to Finny's conception of sports (in which they are purely good and no one ever loses).