The "if" is significant. If you work on weapons that turn out to be used to stop legitimately bad actors with minimal collateral damage, then your hands are as clean as anyone's can be in war.

I don't generally believe in black and white, so I'm making no claims that anything is 100% ethical, 100% unethical, 100% blood on your hands, etc. I think it's possible for good people to work on weapons and still sleep soundly at night. I also think it's possible for good people to work on weapons and end up regretting the consequences of that choice.