It is, of course, almost an adage whilst being wholly true, that life feeds upon life. We may argue about its efficacy, its ethics and even the desirability of the facts but none-the-less it stands that to live, one thing must predate another. You can take this a bit far by suggesting that microbes that derive sustenance from the elements in the air are also feeding on life because of the convoluted way creatures break down in their constituent parts and then merge with gases.
The real discussion though centres around how far we are to accept this fact of life as a dictate to our own actions. Given that most of life may be supposed not to think deeply about ethics, it is given to the human being to argue amongst themselves. Some accept our natures, some revel in them, and some dissociate themselves from this way of living in as far as a higher animal is able to. And there, as they also say, is the rub. For to live we must kill something to derive its energy giving elements which come to us a sugars, carbohydrates, proteins and vitamins and so forth.
This argument has been going on for millenniums, and won’t ever totally go away because we know that killing is not an ethical action and our civilisation has been a continuous debate upon ethics. Feeding both our religions and our laws.
But then even the Universe may recycle everything to maintain its energy levels, so we should not be surprised if we never reach a conclusion.