This is not a good argument.

Vegans base their line on a very easily defensible ability on behalf of the victim - sentience.

If there’s no sentience, there’s nobody within to experience the pain and fear, and there is no victim.

That said, even if you granted that every blade of grass and kernel of corn was fully as sentient as a human being, that would only strengthen the argument for veganism many times over as animals act as inefficient intermediaries for those plant calories, burning most of them and leaving only a small fraction in their meat. You’d kill far fewer plants by eating the plants directly.

Finally, to your other point, many humans die horrible deaths - whether in global poverty, war or of various types of disease, cancer and dementia in the wealthier countries. That of course does not justify serial killer cannibals who put a bullet in the back of their victims’ heads on the basis that they’re giving them a “humane” end and likely saving them a large amount of future suffering.