When it comes to vegetarianism vs meat eating the costs to animals is not always very clear cut. While vegans can often be quite sure of themselves when eating a bowl of oatmeal there are other consequences that they may not have considered. Growing crops may kill more animals and living creatures than pasturing cows in a field. Here is why. Large scale farming involves the use of heavy machinery and vehicles that will run over many small animals in the fields. Groundhogs, mice, voles, snakes just to name a few. Grain farmers will also poison millions of mice periodically, which will also result in their offspring perishing. Pesticides used on non organic farms will also kill billions of insects. Using a field to grow crops will also drastically alter the ecosystem and natural plant life while using the same field for pasture will largely preserve those things.
Not so black and white is it ?
Consider that this situation only involves grass fed cattle though. It's easy to see that factory farms for chickens are far more harmful than that cattle pasture. And some farms do not use machinery and instead just have people harvesting by hand. My personal philosophy is about minimizing the damage to other living creatures. Vegetarians would be well advised to think more closely about how the food on their plate came to be there and make their food choices accordingly. I got some great stats on the question of whether crops or raising livestock is responsible for more harm to animals here. Sometimes you have to pick the lesser of two evils.