Alejando1's point was that Bentham expected everyone to be a "defector", in your terminology, and but that lawmakers should be given selfish incentives to maximize the sum of everyone's utility. Although it is unclear to me who could be motivated to ensure that the lawmakers' incentives are aligned with everyone's utility if they are all just concerned with maximizing their own utility.

Also, as long as we're talking about utilitarianism as described by Bentham, it's worth pointing out that by "utility", Bentham meant happiness, rather than the modern decision-theory formulation of utility. According to Alejando, if I understand him correctly, Bentham just sort of assumed that personal happiness is all that motivated anyone.