It's not quite that abrupt. I used to work for a defense contractor. I started as a rather naive young engineer. I worked hard for many years on projects that were not weapons systems or even fielded systems, mostly R&D and advanced technology. It was very cool technology. But over the years I gradually became more and more aware of the kind of systems my employer was developing. And although I did not work directly on those systems I knew that some of the technology I was developing would eventually find its way into future versions of those systems. Colleagues would always say "We aren't building bombs so it's ok." But what we were building were key subsystems used for targeting. I eventually left for the non-defense world.

So it isn't that friends suddenly became ex's overnight. It is after coming to the realization that over time they either had sold out their ethics in order to make good money working on cool technology regardless of what it is used for, or that they really bought the indoctrination that what they were doing was important patriotic work. As for the latter, I heard that stuff a lot, and I always thought it was a bad sign that the boss had to kept telling you that you are doing important work. You weren't privy to much beyond your own compartmented subsystem but over time you could eventually piece together the bigger story. As for the former, that's similar to the non-defense world where a lot of smart engineers are developing shady stuff, except that it isn't resuling in thousands of people in far away lands being killed week in and week out as a result.