It's the same with weapons design and scientists, or battlefield medicine and doctors. At some point the challenge overcomes the ethical qualms. Or, in some cases, the need to eat becomes pretty overwhelming. Morally dirty but technically difficult jobs often pay really rather well, partly because many talented people are turned off by the field.
Real example: a good friend of mine was in my physics class at university. He was a pretty peace-loving, anti-establishment, idealist type like all of us at that age, yet he's now a government employee spending his days inventing ways to make ICBM warheads invulnerable to anti-missile systems. It's cutting-edge materials science stuff, and he loves it. (As a cold war kid myself I'm slightly ashamed to say that I found the whole thing deeply cool and I'm quite jealous of his job.)