Rawls's approach seems to be to say, morally, you don't know in advance which individual you're going to be, and therefore you should arrange society so as to maximize ... to maximize, not the average or expected-value happiness, but the minimum happiness, of the individual you might end up as. So a slight comfort to the worst-off person is worth imposing a significant penalty on everyone else, as long as the penalty isn't so severe as to make someone else worse off than the one person. As if Rawls is extremely risk-averse and thinks this should be the organizing principle of society. If someone took that principle and ran with it, it would probably justify huge amounts of destruction. [1]

Also, consider this interpretation: "The best way to solve everyone's problems is to create the technological singularity, at which point we truly can fix the genetic problems that cause the worst-off person in the world to constantly feel pain; and therefore anything that most efficiently and reliably achieves the singularity is the best long-term option for the worst-off people, even if the short-term aspects of the most efficient approach involve e.g. cutting off all aid to the disabled and euthanizing anyone who's too old to work, nuclear preemptive strikes against "rogue states" that might endanger progress, wiping out all animals and plant species that take up space we could use more efficiently some other way, etc." That obviously carries its own potential justifications of mass destruction. I'm not saying all of those are necessarily the most efficient or reliable approach—e.g. the nuclear preemptive strikes might lead to a worse backlash from the surviving countries—but some of them might be. More importantly, some people might be persuaded that these are the most efficient or reliable approach, and be mistaken. That is a fundamental problem: even if we all agree on a certain set of goals (which is a big if), reasonable people can have very different, mutually exclusive ways that they think are the best way to fulfill them.

Note that "promulgate my philosophy/plan to everyone, and kill all the heretics who don't accept it" is one potential step that could be added to any plan; and it seems like if you did manage to carry it out, it would guarantee success; and if your philosophy/plan is big enough and important enough, it might seem like it's worth doing (and thus it will probably tempt some adherents of every philosophy ever), unless you have, say, principles that say you should never do a thing like that, no matter the apparent benefit.

Raymond argued[2] that such principles can be justified within utilitarianism: you can never be sufficiently certain you're right. If you're right and you kill the wrongthinkers, maybe this gets you there a little faster than if you used more peaceful methods: a slight benefit. If you're wrong and you kill the "wrongthinkers", the consequences are extremely severe. It's worth killing the wrongthinkers if and only if the chance you're wrong is smaller than the ratio of "slight benefit" to "extremely severe". The principle would be, you're not allowed to decide that the chance you're wrong is that small.

That might work. But one way or another, I think you need to have principles that prevent you from knowingly doing these horribly destructive things. I would say anyone without such principles is not civilized; and, from recent revolutionary history, it seems those with strong moral philosophies and no such principles may be dangerously uncivilized. (Note: Stepping outside of civilization may be appropriate in some circumstances. But then you have no right to complain when good civilized people arrest you and punish you.)

[1] This argument is expanded on here (not by me): http://www1.cmc.edu/pages/faculty/ASchroeder/docs/RawlsMaxim...

The citation says: "Surely Rawls wasn’t crazy, so we can conclude that this isn’t how he meant to pursue the argument." Which I don't think is very helpful.

[2] http://esr.ibiblio.org/?p=4878