This is a common argument people use against veganism. People claim that no one is perfect and therefore vegans cannot be telling others not to eat meat because they too are not perfect. For example, when plants are grown, insects are harmed when tractors harvest the plants.
Vegans seek to minimize the pain and suffering inflicted on other beings. No vegan is perfect as you could argue even just breathing can harm a living being due to the butterfly effect, but most people would agree that just because it's impossible to never harm living beings, there should still be attempts to mitigate harm e.g. animal cruelty laws and laws that protect humans e.g. laws prohibiting murder, rape, etc.
Many people justify eating meat saying no one is perfect therefore they should be allowed to kill animals, but do any of these people use the same argument to justify abolishing murder legislation?
The definition of veganism as provided by the Vegan Society is as follows: "Veganism is a way of living which seeks to exclude, as far as is possible and practicable, all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose."
The way I see it, animals are today's slaves. Only hundreds of years ago there were groups of humans deemed subhuman and subjugating them was seen as natural by social darwinists. Today humans widely believe in a social darwinism applied to animals, that animals are lesser beings. My hope is that as humans evolve animals will gain rights as did slaves, women, etc.
Inefficiency of meat production
Something else I should point out is that it is true that growing plants can kill insects, but plants grown are used to not only feed humans but also livestock. Growing plants kills insects, but keep in mind that livestock, in order to grow, are fed plants. Figures vary, but it takes between 2kg to 7kg of plant feed (mostly soy and corn) to produce 1kg of meat. Hence more plants must be grown to feed meat eaters than to feed vegans due to energy lost. The greatest energy loss is found in warm-blooded animals like cows as the chemical energy in plants must be converted into heat to keep the animals' blood warm. In cold-blooded animals such as fish and even insects the feed to meat ratio is lower. Nevertheless, it is never as efficient than if the "middleman" is cut and the human consumes the plant directly.