Eating those plants directly would nourish you better for longer. So, disregarding animal rights, a vegan diet is less harmful to plants. But it is not wrong to eat plants. An animal's life isn't given value just because it is alive. It is given value in ways human life is given value.
Higher animals can feel love, pride, curiosity, and triumph.
Some have a sense of humor. Usually they can show these emotions, too. Mammals nourish their offspring with love just like we do. Many animals mourn their dead mates and family members, too. These things give life value.