Ask HN: Why is there a stigma around working in defense-related tech?

I've noticed people have a low opinion of people who are in military-related work. I know the obvious reason is that the military tends to involve killing sometimes, but that's not what they do all the time. For example, the USA military also has a lot of programs for food distribution, girls' education, and preserving human rights for minorities in all the places the USA was involved militarily. People don't have similar disdain for working at advertising corps like Google and Facebook for the killings they do indirectly. People brag about working in these ad corps.

I thought about this because I am currently working on a pretty advanced drone that I want to sell to the DoD. My friend argued that I am in the "death industry," which pisses me off. Drones could be used to help distribute medicine in remote areas, or for surveillance. My drone is for surveillance only—it has a self-detachment feature (SDA) that is activated when the drone is attacked, but other than that the drone is for surveillance only.

My friend still insisted what I'm doing is bad and equated my work to some sort of mercenary, as an analogy. Granted, my friend is on the far left of the political grid, so maybe this is an exceptional situation.