"You shall not eat anything that dies of itself; you may give it to aliens residing in your towns for them to eat, or you may sell it to a foreigner. For you are a people holy to the Lord your God" (Deuteronomy 14:21).
Does this mean we shouldn't store bought meat, since this is already dead? (I am trying to decide whether or not to be a vegetarian.)
This is a good example of how important an accurate translation can be in understanding the essence of a biblical passage—particularly in the early books of Hebrew scripture. There is an important difference, I think, between "meat that is already dead" and "anything that dies of itself."
What is prohibited here, I think, is the consumption of carrion—the meat of animals who have died in natural ways that cannot be sanctified before the life is taken. As with most of the dietary laws in Hebrew scripture, this passage is rooted in practical considerations. As we like to say to kids picking food off the floor, "You don't know where it's been!" Only when the process of butchering is done under prayerful supervision could people in those ancient days be sure the meat was safe to eat, free of poison or decay.
As to the question of becoming a vegetarian, that has to be a very personal choice. Jesus teaches that "It is not what goes into the mouth that defiles a person, but it is what comes out of the mouth that defiles" (Matthew 15:10). So the question of eating meat or not is about what feels right to you, and your inner guidance will unfailingly make you aware of your efficient choice. What's important is to guard against making one choice "good" and the other "evil." Follow your own guidance without judging others who may be making a different choice.