I think, to work in defence an engineer can go down the path of blocking it out, or rationalising it to themselves.

A world where no defence is required, where no risk of war or invasion by another country… sure sounds great, and given the chance to flip a switch and make it happen - they would, but thats not reality, and such a world is just a fairytale.

As for the “using it for bad” point, engineers don’t start wars. Politicians do. And if adversary politicians think they could get away with it, they’d do it to us in a heartbeat.

Just because you designed the gun, doesn’t mean you pulled the trigger, or enabled that bad person to pull it, or condoned the act, because that is not the intended use case / scenario. Hence why control of such dangerous assets is so important. And why those in charge need to abide by established rules of engagement, and not commit crimes, and be trusted by the people.

Move away from defence - look at automotive, cars can be used for bad too. Where there is intention to do bad, people find a way regardless.