Like I said it is just one vicious circle and you said it again when you stated "The reality is that for one thing to live something else has to die." That is completely true. The cancer analogy does work but since others have said bacteria deserves to live then surely cancer is just an entity that is trying to live its life. If we happen to be it's choice "habitat and food source" then that is that. But like you said for one to survive something needs to die, so we kill it. And that being said if reptiles need to survive is it then ok for prey items such as a cute puppy or kitten to be eaten?
Also I state previously if you want us to live off lettuce we will destroy more land doing that. It can't be done and it can't be done so that we only live off meat. There has to be balance there.