Absolutely correct.

: Y'know, I'm kinda irritated by this rationale that the Spartans were
: justified because the Covenant came. If Spartans were bred to stop a
: genocidal foe, I would perhaps accept such justification on the principal
: of saving humanity. But they weren't. The Spartans are humanity's
: deadliest weapons yet.

You should be irritated by that rationale.

: I think of it in historical terms. The atom bomb was created to kill other
: humans with indiscriminately immense power. They perhaps saved the lives
: of millions of Japanese and the Alled troops who invaded the mainland.
: But, they also killed and injured hundreds of thousands of other
: individuals. To this day, we have been on the brink of nuclear catastrophe
: because we couldn't settle our differences like civilized people.

I think you're a bit derailed on this example. I'm not sure it's the same moral issue as kidnapped child soldiers. The A-Bomb was simultaneously being researched by many countries. It doesn't differ from any weapon at any time when one side has the a clear and unchallenged advantage.

Thankfully, our world had enough war for awhile. Thankfully more countries acquired the bomb. While I certainly don't want additional proliferation and even more countries acquiring such weapons, equilibrium is a moving target in a changing world.