I can't speak for the person you're replying to, but some people adhere to a consequentialist worldview, where intent does not matter (or matters relatively little). It's the same outlook that creates the need for good Samaritan laws. I don't agree with it in whole form, but it should be recognized as a valid philosophical framework.

I understand that, but I have a hard time believing a vegan diet could possibly harm animals more than an omnivore diet. Like it just sound so absurd and I run in to a lot of people online that just seem to want to bullshit themselves about veganism for some reason. Like the guy that said I was killing deer by eating only plants because it means more tractors and tractors can run over deer… that seems to me to be a shoddy leap of logic, not a genuine point I should consider.

>bullshit themselves about veganism for some reason

I think this is also the most likely reason because humans try to rationalize cognitive dissonance. But in the nature of the HN guidelines to try and take the strongest possible interpretation of someone's comments, I'll try to reframe it.

Suppose someone has a foundational principle that all animal life is sacred. Also suppose that person is a consequentialist. Following the first point, the life of an insect is every bit as valuable as the life of an elk*. It's easily conceivable to me that farming, in the current form that most of use get our food, kills many more animal lives per calorie than eating the elk. If one is a consequentialist, it doesn't matter if there was the specific intent to kill those thousands of insects (and fawns, and fish from runoff etc.), it only matters that my choice led to their deaths. Compare that to the hunting of the elk, where it's conceivable that only one life was lost. From that perspective, a vegetarian/vegan diet could be considered more detrimental to animal life.

(I specifically chose elk to avoid the complication of how much farming goes into livestock feed, but hopefully you understood the point)

If you're like me and don't necessarily agree with the above framework, it still highlights that most of us have unarticulated assumptions that may not hold as we go about our daily choices. E.g., even if people claim that they think all animal life is sacred, their actions display an assumption of a hierarchy to that value.

I don't know if this is helpful, but this 2003 research estimates about 6 animal deaths per acre of agricultural harvest, or about 7.3 million animals.[1] That puts it about on par with industrial animal consumption. But granted it seems much more difficult to have accurate estimates.

I think part of the problem is that most people in industrialized economies are extremely far removed from the food supply. I don't think it's super common, but also not unheard of for deer (especially fawns) to be killed by harvesting machinery as they bed down in fields. Considering hunting a deer may yield 60-70 lbs of meat, it might be possible to claim that reduces total animal deaths. Now I don't know how you'd arrive at a final number because most people are omnivores and still eat plants, but I don't know if I'd immediately dismiss their claim as absurd.

I see from your profile that you work in the farming machinery business so I'd be curious if there are any specific mitigations you would consider to alleviate this problem?

[1] https://link.springer.com/article/10.1007/s10806-018-9733-8