When doctors or nurses use their knowledge of anatomy in order to torture or conduct medical experiments on helpless subjects, we are rightly outraged. Why doesn't society seem to apply the same standards to engineers?
When a doctor tortures a patient there is a direct cause and effect from the doctor's actions to the pain and suffering of the victim.
When an engineer designs a weapon, he's not actually causing the pain and suffering. Once you get away from "complete responsibility", the rest is easy:
1) If I don't do it, someone else will
2) I need to feed myself and my family
3) It'll only be used on the bad guys
4) It helps protect my country
5) It's the user's responsibility, not mine
6) The boss thinks it's a good idea
7) It has significant non-evil uses
8) No one will ever know it was me
For a concrete example, consider the Collateral Murder video from a couple of years back. Who was responsible for these deaths?
The helicopter pilots got the go-ahead from their commanders, the commanders [probably] got the go-ahead from intelligence services, the services made the correct decision based on the information they had, and the information was somehow "wrong".
Who's to blame for the collateral murder incident? By deftly distributing blame among many players, it changes from personal responsibility to "a failure of the system", or "a tragic accident".
For a second example, consider Bush's Iraq war: he was on TV stating that he had convincing evidence of WMDs in Iraq. A couple of years later it came out that the intelligence services had never said this and tried to convince the president of the opposite. Bush's response was: "We [the administration] didn't get the message". (Note the use of "we" in his statement.)
Who's responsible for the war? The President says he got bad intelligence, the intelligence services say they never gave bad intelligence. It's impossible to lay the blame on someone, it's a "failure of the system".
But don't worry, the problem is fixed - it'll never happen again.
(Epilogue: The Gulf oil spill was largely enabled by failures of the Minerals Management Service, who is responsible for overseeing the safety procedures of off-shore drilling. The problems were largely fixed by renaming the service to Bureau of Ocean Management. The problem is fixed, now we won't have any more disasters. Sorry about that...)