Being uncertain about one's uncertainty doesn't mean you shouldn't act according to your moral beliefs, the uncertainty can go both ways. You still have to assign probabilities in the end.
Assuming you attribute the same importance to reducing suffering, no matter in what type of being it occurs, it still wouldn't make sense to "shoot people left and right". People who care a lot about animals are already being branded as "extremists", you'd have more impact by going about it with a more thought-out strategy like e.g. movement-building and / or high-earning and donating to the best outreach organizations.
But then this brings in other considerations--if he can mildly hurt other people for vegetarianism, then others will also be able to mildly hurt him for their own pet causes, for instance, and things may be overall better off if nobody is allowed to hurt others for their causes.
What's the difference between a pet cause and a normal cause? Majority rule?
You still have to assign probabilities in the end.
True, except that one's certainty about one's certainty is typically not a large number. If you think that killing one person justifies creating 1001 vegetarians, are you even 50% certain about that number 1001?
it still wouldn't make sense to "shoot people left and right".... you'd have more impact by going about it with a more thought-out strategy
Yes, but "it makes sense to shoot people" is shorthand for "the moral reasons that normally mean we should not shoot people do n...
I notice that a large fraction of Effective Altruism people are vegetarian. This makes sense: in general Effective Altruists take moral issues seriously, even when that means changing your lifestyle. I'm not sure it's a good balance, though.
One way to think about this is to convert it into money. How much would I need to be paid to give up eating meat? All animal products? How much money would I need to spend on myself to be about as happy as I would be with less money but continuing to eat animals? I'd probably be willing to go vegetarian for about $500/year, vegan for maybe $2000/year.
It turns out you can probably pay to convince other people to be vegetarian much more cheaply than that. I estimate the cost of a vegetarian-year at $4.29 to $536 while Brian Tomasik estimates $11 with better methodology (which I looked at). This is by placing ads on facebook for a site where people can watch an animial cruelty video and ideally become vegetarian or vegan.
If you would get more than $11/year worth of enjoyment out of continuing to eat meat, why not give $11/year to convince someone else to not eat meat for you? Or give $50/year and be on the safe side?
(While you're giving money, you should probably give it to the organization that you think will do the most good with it, which I think is probably one of GiveWell's top charities. The nice thing about money as opposed to actions is that it's easy to redirect.)
I also posted this on my blog.