Not breaking the first precept vs. developing compassion
Compassion is a wholesome quality, but compassion cannot take you to Nibbana. We use the practice of compassion to counter cruelty in the mind. But the goal of Buddhism is not about dedicating your life to the cultivation of compassion. If that is the case, you will have nothing else to do in life other than looking for beings in misery and trying to help them. How can you even eat, drink, wear clothes if all your actions are dedicated to compassion? There are always beings who do not have food, water etc. You should give all your food to them and starve to death yourself, if compassion is all that you should cultivate. 

Vegetarian food production causes much killing as well. So why not starve to death to save any possible loss of life? This kind of thinking comes from Jainism. Buddhism does not fall into these extremes. The goal of Buddhism is the end of suffering and there are other more important qualities that one must cultivate to achieve that. Being vegetarian is not one of them.