Easiest one that I can come up with is defense contractor. Some people consider it unethical to be designing weapons.
Once got offered a job in the defense sector. Justified it to myself on the basis that a nation needs to defend itself and I'm not a pacifist. Then it turned out the job was developing arms not for my own country but for a corrupt, human-rights abusing state that my country is in an alliance-of-convenience with.
Fact is, much of the defence sector is no longer about 'defense' its about earning foreign exchange by selling stuff to despots that (at best) will rust in a desert somewhere and, at worst, will end up being used against innocent civilians or one's own country. The ethics of that seem very dodgy to me.
