Vegans kill "some animals" by accident on roads (or by other means), so why not kill just "some animals" as part of a half-measure like Meatless Mondays?
Humans are killed in car accidents every year. Humans die in factories. So, if you use any product ever, some human might have died in some factory somewhere to make that product. Is murder now moral because humans are killed by accident in society?
Furthermore, can someone use this argument if someone finds them torturing an animal in their basement? What if they cut the fins off of a dolphin and throw it back into the water to suffer and die? Could they then just throw their hands up and say "Welp, animals die in modern society, waddayagonnado?" This seems like a rather poor defense in any other moral context.
Let's consider the two major schools of thought in ethics: 1. deontology (i.e. that moral imperatives towards the subject are what you need to consider in order for an action to be considered ethical) and 2. utilitarianism (i.e. that the maximum possible well-being is to be aimed for in order to be considered ethical):
Taken to its logical conclusion, this is just a position that if some background number of animals die, whether in harvesters, on the roads, or whatever, then there's no point in ever taking any accountability towards attempting to reduce that number even if it's by a huge amount by doing very little. That means if a single animal ever dies by human cause ever, you can always just throw up your hands and say "Welp, we couldn't protect that one animal, now that means that I get to dig into these chicken wings", which just sloughs off responsibility and is not a reasonable position because now almost anything is justified. "Can't stop all theft? Theft is now moral." It's not a coherent ethical principle.