So, symbolically, Bobby is forced to the ground and treated like an animal, and when he rises he, and the group, have accepted this new animalistic nature. All, of course, except Drew. Drew is the last moral holdout among the group, arguing that the men should not bury the slain attacker and should do the right thing and go to the police. He is a voice of peace and reason, and when he eventually falls from the boat into the water (presumably shot), it signifies the final moral collapse of the group. Not long after, Ed kills the second mountain man who the group believes is stalking them. In the cases of both the mountain man and Drew, the men weigh down the bodies so they will never resurface - in the case of Drew, the action is symbolic in that it shows the men's former sense of morality will never be resurrected. Surviving in the backwoods has required them to abandon their values, which shows how strong the distinction is between the two societies.

Throughout the movie, Lewis serves almost as an escort for the other characters into a different moral space. He reminds the men, from the beginning of the movie, that the laws...

He fancies himself a survivalist, which is a role that none of the other characters claim. And, in fact, it is Lewis who ushers the men further away from their more civilized sensibilities - first by shooting one of the attackers, and then by suggesting they bury the body. The men begin to adopt Lewis' survival-of-the-fittest mentality, which is most evident when Ed kills the mountain man and when Ed and Bobby take the lead in deciding to sink Drew's body.
There is a clear lesson in Deliverance about the difference between city and rural people. Boorman goes to great lengths to show that these two groups of people have completely different values and, in a sense, different definitions of survival. The main characters define survival, at least at the beginning of the movie, in terms of financial wealth and material comfort, where the rural characters have a more basic sense of survival. The main characters come to rural Georgia looking for a taste of something simple, but instead find a world much more complex. It is a world where they are not revered, but, instead, are mistrusted, despised and even physically abused. There are two separate worlds and, in the end, when the men promise the local sheriff never to return, it is a fitting symbol that these worlds should not - and can not - be integrated.