Yes, this is just what I mean.
The idea that veganism is better environmentally is bollocks. It's possible at a scale that can actually feed people only in an industrial farming, monoculture, petroleum based agricultural model, and that's why it comes out a bit better than that same model with animals. Which is to say - neither efficient nor sustainable.
The most sustainable and efficient agricultural models are always mixed farming models - because farm ecosystems, like natural ecosystems, have the most synergies and interactions when they include both plants and animals, and once you seriously try to reduce reliance on petroleum animal power of one kind or another becomes important on a farm.
Even in the odd historical pocket of veganism - and they are vanishingly rare - you will find there were not vegan farmers. Veganism in these scenarios was a privileged position that depended on other people getting their hands dirty. Because farming with only vegetation isn't efficient without oil based fertilizers.
So no - not everyone "knows" that not eating meat is better for the environment.
As for killing - all of us, whatever we eat, survive because of, and are in fact physically constituted by, dead organisms. All the ecosystems that sustain us are made up of both plants and animals, and are dependent on those existing in a cycle of life and death. Populations in nature are not some idealistic pastoral situation where every animal dies of old age, quite the contrary, it has always been usual for large parts of animal populations to die young. Which in general is not more awful than the few who live to old age and die of starvation, exposure, or an overload of parasites. Animal husbandry, when done properly, does not offer a worse life than nature for animal populations. (Not to get into the fact that farming monocultures of plants are really really terrible for animal populations of all kinds.)
I would submit that it is much better to understand our place in these systems, our dependence on other life, and respect it and look at it clearly, as opposed to trying to fool ourselves into thinking we can somehow exist outside of them so we don't need to feel our intimate, and in fact visceral, connection to other life in a way that feels uncomfortable.
So again, the whole "aw, you know you would be so much nicer if you just let those farm animals live out their life at the animal rescue, you just won't admit it cause you like the taste" isn't something I recognize, and I actually think it's shallow when it comes down to it.