The line is simple, do you have the choice (personally or influentially) to avoid that? If yes, you draw the line. Can you avoid paying taxes? No, can you influence not to be used as you mentioned? You try. Can you avoid a job that obviously will use your expertise in questionable ways? Yes. The right and wrong is clear, people who try to muddy it they just seek justification.

I personally turned down multiple offers/work when it comes to drones for example, I designed stuff back in 2020 that some companies now just knew about it and its potentials, but when you meet with them and discuss what they are trying to build, you know that whatever they are after can be used in bad faith if the company decided to do so, do you sell your soul for the cash or do you prefer to sleep well at night?

The only exception is whatever you built has no direct or obvious bad faith, you build a library, a gstreamer addon, etc., that can get a pass.