What was Gene unable to say in his story?

From the text:

“Well,” I replied in a stronger voice, “first I stole all his money. Then I found that he cheated on his entrance tests to Devon and I blackmailed his parents about that, then I made love to his sister in Mr. Ludsbury’s study, then I …” it was going well, faint grins were appearing around the room, even the younger boy seemed to suspect that he was being “sincere” about a joke, a bad mistake to make at Devon, “then I …” I only had to add, “pushed him out of the tree” and the chain of implausibility would be complete, “then I …” just those few words and perhaps this dungeon nightmare would end.

But I could feel my throat closing on them; I could never say them, never.