In 'Deliverance' what is ironic about the story ends?

I suppose that what was supposed to be a male bonding trip turns out to be a trip of death and disaster. All of the men head into the woods in order to take a man trip down the river. The irony is that only one of the guys on the trip, Lewis desires to be a man of the mountains and prove himself able to survive. The other three just want an escape from their daily lives and that is why they say yes to this trip.