That's a rather overgeneralized mischaracterization of the ethic amongst those working in the Defense industry. You pre-suppose that what they are doing is wrong.
Here's the way I see it. Show me a largish country on this planet which does not have a military to defend itself, and I will consider your hypothesis that defense engineers are not needed. The fact of the matter is that countries which subscribe to the total pacifist code of ethics simply cease to exist because they're invaded and taken over by other countries. So if the choice is between nonexistence, and designing stuff to help protect and keep alive what I think is a pretty decent socio-political system, hell yeah I'll help design the weapons.
I wasn't attempting to characterize all, just some. And I have no idea where you get the idea I had a hypothesis that defense engineers aren't needed; I've been one myself for the past seventeen years.
There is certainly a sort of "mercenary" ethic amongst many defense engineers. As long as there are soldiers willing to pull triggers, there will be engineers willing to design the guns. As well as simple game-theory type reasoning - "I can take the pay for this job; but if I don't, they'll find someone else who will." I get the feeling the article author doesn't know and didn't really talk with any longtime defense engineers - professors can be quite removed from that world.
And this is to say nothing of the defense engineers who are actually gung-ho about their work.