This is your typical argumentum ad absurdum, with the bonus of including a false equivalency. Is it improper behavior to put poison on your own property due to the fact that it might kill a thief? Imagine the following: Your neighbor has been stealing your food and refuses to stop. You purchase potatoes that were somehow produced without contributing to any animal cruelty or death at all. Since you exclusively own the food and are immune to the effects of Chemical-A, you decide to put Chemical-A on the potatoes. Your neighbor trespasses on you property and steals potatoes, eats them, and then dies. What behavior was improper? Is stealing proper or improper? Is defending your property that you're depending on for food to survive the same as purposefully dominating innocent sentient beings with the intention to kill them in the name of fulfilling taste preferences? Are thieves dying from eating the poisoned food that they stole the same thing as innocent beings needlessly being dominated, raped, exploited, and killed to meet a market demand that is based merely on arbitrary preferences and or delusions? As technology gets better and better, humans will likely figure out ways to kill less and less insects while accomplishing the same, if not better things, especially if that remains a market demand; but for now, some will die from trespassing and trying to steal food.

Veganism is a lifestyle that seeks to exclude, within what is practicable, animal exploitation and cruelty to sentient animals, and since farming food for humans in soil with modern technology wihtout killing any insects is not practicable, it therefor does not violate vegan principles to accidentally kill insects while trying to grow food, or to use insecticide to defend your property (food) from being stolen.

The majority of the pesticide used in the agricultural industry is for crops that are then fed to livestock. Livestock eat significantly more plants annually than humans, and they are primarily fed crops that are sprayed with more, and worse, pesticides. Most Vegans make efforts to buy organic groceries that use less insecticide or at least insecticides that are less detrimental overall.