I think it was good that he didn't kill the guy at the end because it makes a contrast between purgers who do it for the thrill, and purgers who really see the purge as an outlet for their "primal" instincts AND purgers who are really just good people who are pushed to that point. I think born out of the very dark dystopian-country, it was a good choice to have a semblance of hope in the form of the rebel faction, and even moreso, the guy who ultimately chose to forgive. The NFFA justifies the purge as a means of reducing crime: It allows people to vent their frustrations in government-endorsed killing spree, and it forces people to be nicer to each other as to avoid being marked for the purge by other people in the community. In truth, it was a way for big defense companies to make money, control the lower-class populations, and further consolidate the wealthy's power over the poor. (because they can't afford protection)--I cheered so much when they dude (I forgot his name) was killing those rich people who thought they'd have a little fun with their "prey".