And there are oystrovegans. Who eat vegan food and oysters. Because oysters have the mental capacities of venus flytraps. Incidentally I think this would be a good, mostly sustainable and cruelty-free diet, weren't it for the price of oysters.
Plants are autotrophs and don't eat other things. Animals are heterotrophs and do it other things. Applying the Golden Rule in reverse, it's fine to do to lifeforms what they do to others.