Animals and plants are both living beings, so why is killing and eating plants OK?
Thank you for your letter. Not eating animals because it's painful to them is only part of the reason that we don't eat meat. There are several reasons.
One is that Lord Krishna eats fruits, flowers, grains, milk and water...and since we offer everything to Him, we offer only those things. Secondly, we have to eat something. And the Srimad-Bhagavatam explains how one living entity is food for another, so in order to sustain ourselves, we eat those things which will suffer the least.
Most fruits are picked without harm to the tree that produces them. Most vegetable species produce many vegetables - picking them only strengthens them; the plant continues its life cycle and eventually dies. Grains are harvested only after the plants have reached maturity.
Most important, there is no bloodshed or violence. If one thinks of one's own emotional response to fear, it is easy to determine that many chemicals enter into the bloodstream when one is afraid. When an animal is slaughtered, such a fear reaction is there, and then the person who eats that meat is eating those chemicals that, in turn, lead to heightened fright and flight response, tension and all kinds of disease.
The human body is far more suited for a vegetarian diet. We don't have large, sharp canine teeth for tearing meat, therefore we have to cook it and spice it in so many ways before it can be digested. Our digestive tract is very long and convoluted; difficult-to-digest foods take a long time to pass through the digestive tract. This food can then putrefy and cause disease. Carnivorous animals have very short digestive tracts and heavy acid secretion to digest raw meat.
Ultimately, if we're trying to be devotees, we want to eat food that enhances the mode of goodness, rather than passion and ignorance. We want to eat food that brings health and good emotions to the body and at the same time can be offered to Lord Krishna for His pleasure so we can take the remnants.
This is just a brief summary, for more information you can look in the beginning chapters of "The Higher Taste" cookbook. There's more detailed information there.