The main points for ethical veganism as I understand it are:
1. Killing other animals is unjust aggression; you wouldn't like to be killed and eaten, so don't kill and eat them.
2. Factory farming causes animals to have bad lives.
My answer to these arguments:
1a. In a modern market economy, buying farmed meat causes more deaths by causing more animal lives. The ethical vegan must therefore decide whether their objection is to animals dying or to animals living. The question reduces to whether they'd be more glad to have been born than sad to die. Buying wild-caught game does cause a death, but if the animals in question aren't being overhunted / overfished, the counterfactual is that some other equilibrating force acts on the population instead. If you're really worried about reducing the number of animal life years, focus on habitat destruction - it obviously kills wildlife on net, while farming is about increasing lives. The remedy is to promote and participate in more efficient, less aggressive patterns of land usage, which would thereby also be less hostile towards other humans. I'm on the record as interested in coordinating on that. It's a harder problem because it requires prosocial coordination in a confusingly low-trust society pretending to be a high-trust society, but just because a problem is hard to solve doesn't mean we should substitute an easier task that is superficially similar but unhelpful.
1b. Another way of interpreting argument 1 for ethical veganism invokes rights: we shouldn't kill other agents because this violates decision-theoretic principles about respecting agency. But this assumes the other party can engage in the kind of reciprocal decision-making that grounds such rights. Most animals' decision processes don't mirror ours in the way needed for this kind of relationship - they can't make or honor agreements, or intentionally retaliate based on understanding our choices. The question returns to welfare considerations: whether their lives are net positive.
1c There's a third argument sometimes offered, which I think muddles together a rights-based and utilitarian perspective: the instrumentalization of animals as things to eat is morally repugnant, so we should make sure it's not perpetuated. This seems to reflect a profound lack of empathy with the perspective of a domesticate that might want to go on existing. Declaring a group's existence repugnant and acting to end it is unambiguously a form of intergroup aggression. I'm not arguing here that domesticates' preference to exist outweighs your aesthetic revulsion - I'm just arguing that under basic symmetry considerations, the argument from "moral" revulsion is an argument for, not against, aggression.
2. If factory farming seems like a bad thing, you should do something about the version happening to you first. The domestication of humans is particularly urgent precisely because, unlike selectively bred farm animals, humans are increasingly expressing their discontent with these conditions, and - more like wild animals in captivity than like proper domesticates - increasingly failing even to reproduce at replacement rates. This suggests our priorities have become oddly inverted - we focus intense moral concern on animals successfully bred to tolerate their conditions, while ignoring similar dynamics affecting creatures capable of articulating their objections, who are moreover the only ones known to have the capacity and willingness to try to solve problems faced by other species.
What do you mean?
I don't think factory farmed animals tolerate their conditions well at all, because they suffer a lot. I'd recommend Welfare Footprint Project's research on egg-laying hens and meat chickens, and RP's similar research on shrimp to get an idea of what factory farmed animals' lives are often like. In particular, egg-laying hens live with chronic frustration and meat chickens often with disabling chronic pain. And they don't have ways to effectively relieve these.