I can't remember the exact quote, or even the source, but it was along the lines of "For engineers, the biggest [moral] issue with weapons development is that it is
fun".
You can build some really neat things when you have a military R&D budget behind you. The downside is of course
that it actively or passively hurt people.
And then there's the whole calculus-of-death/unintended consequence thing, where in theory, a more accurate missile is better for everyone because it reduces collateral damage. But because it kills less innocents, there are fewer qualms about using it more often. This applies strongly to the current UAV trend - You can politically spin (or just bury in accounting) "Ooops, We lost a $xMM drone", versus actual pilots dying.