does evil exist?

[The Grand Nudger]

You may have goals, but that would be additional to and separate from whatever was necessary.  It makes little sense (logicaly speaking) to aim goals towards or speak of goals as the necessary - as they happen whether you seek them or not.  So, in your example, while heterotrophy may be a necessary evil, going full-on twelve monkeys is not.

That's making assumptions about free will, determinism, etc., no?

Maybe it's inevitable that any species developed enough to have mores will necessarily wipe itself out.

I think I'd call that one a logical possibility rather than a logical necessity.  If it were necessarily true, we would not be here to discuss it. Not just us, but any other social species - each developed enough to have mores, many of which our ancestors...which necessarrily survived such a state of being in order for us to be here. Mores won;t have the opportunity to wipe them out, thus certifying the claim, because they're already gone and mores didn't do them in. Still, it's one of those things where our place in time (and as existent examples of the set) may misinform us. We could posit that mores are deleterious in hss as an axiom of our system and it would create quite a few relationships of necessity. Relationships which wouldn't pan out at a glance even in our lone case. This is the point at which we would have to concede that our axiomatic system doesn't accurately map to reality - so something is either wrong with the axiom or wrong with reality.

What makes heterotrophy a necessary evil in the example is that, given the axiom of our system (eating things is bad), there can be no world in which there are heterotrophs which does not contain this specified evil.