What happens at the end of "The Zoo Story"?

After Jerry has completed his story about the dog, he sets about physically encroaching on his space. He tickles him, pushes him, punches him and tries to get him into a fight over the bench - Peter refuses.

Jerry then produces a knife - the first time in the play that we've been aware he possesses one! He throws the knife to the ground, and, again, challenges Peter to fight for his bench, hitting him and slapping him until Peter, finally losing his temper, picks up the knife. Then comes the surprise:

Peter holds the knife with a firm arm, but far in front of him, not to attack, but to defend.

JERRY
So be it.

With a rush he charges Peter and impales himself on the knife.

Jerry, mortally wounded, thanks Peter and tells him to run away, back to his house and his animals. Peter, after a few stumbling lines, does, screaming "Oh, my God!", words which, as the final lines of the play, are echoed by Jerry in "a combination of scornful mimicry and supplication". Jerry is left alone, dead, on the bench.