TL;DR: there are strong reasons why hypocrisy is bad, despite what Southan says; and veganism is the most politically practical diet to shift society towards the increased care for animals which ethics demands.
As Southan no doubt knows, the argument from marginalized cases is among the weakest in the pro-animal ethics literature—and he still doesn’t manage to knock it down without asserting that hypocrisy isn’t such a bad thing after all.
So let’s look at that vegan premise: that logical consistency is ethically required. Why not just lie to yourself when it’s convenient? “No, I’m not the kind of person who makes others suffer to satisfy my trivial pleasures, but I do pay for animals to be factory-farmed so I can have meat” is a position many seem happy to hold. The answer gets into much deeper philosophy than Southan‘s demagoguery, but it’s very interesting.
There are two reasonably simple answers. The first is that humans, alone in the universe, have to construct meaning and self-worth out of their own lives. If I’m a good surgeon, a good mother, a good citizen, then I can look at these accomplishments and be content with my life in moments of existential crisis. I can say, “My life is worth living; I’m a good person who does good things, and if I keep living I’ll keep doing good things.” But if I’m also a hypocrite, readily lying to myself when I feel like it, and I admit this fact (as Southan suggests in 3a), then this casts everything into doubt. Maybe I’m only flattering myself, I’m actually kind of a selfish jerk, and I can’t trust that I’ll do good things if I keep living! This is such a psychologically damaging position that nobody seriously wants to hold it.
The other answer is that the nature of our own rationality demands that we try to be logically consistent, or else we can’t do things. When we become aware that we have reasons for our actions, we’re faced with the problem of choosing among them. In order to pick a reason to act on, we need to give a reason why that reason is the right one, and then another reason to justify that…and so on. As Kant first articulated, the only way out of this infinite regress is to endorse the principle that our own will is a universally good reason to make a decision. But this claim clearly breaks down if, with Southan, we decide to give up on consistency.
So Southan can’t get away with endorsing hypocrisy. This leaves only his claims about consequentialist details of the least-harm diet. These can be addressed by raising the point—which Southan, of all people, should be extremely familiar with—that eating is a political act. By lovingly describing his reasonably quick killing and butchering of some deer, Southan lends cover to his readers to quiet their consciences as they maintain the horrific factory-farming practices even he doesn’t try to defend. Veganism, in contrast, makes a clear statement to one’s friends and loved ones about our ethical duties towards other animals. Vegan diets are sustainable, easy to practice, and cheap, and they encourage mindfulness of the ways in which we as a society treat animals as we move towards bringing our institutions in line with what the strongest ethical arguments say they should be.