At some point, the realization kicks in that what you're doing is causing harm, no matter how far down the road. When you decide to keep going anyway, that's your moment of sitting at the table with the nazis. Doesn't matter if you're totally removed from the evil doings, you're helping, and you know you're helping. And you're okay with it.

That the definition of a horrible person. Most of these people don't, for example, rape children because they're evil. They do it because it makes them feel powerful, and they don't care about the harm it does. Rarely are there people who's actions are actually inspired by doing harm. It's much more common for it to be done for their personal benefit and not giving a fuck about what happens to others.

The other common form of evil is the banal kind. People who are just doing what they're told, not questioning it. They're told their doing things that are great for their nation/society, but they have horrible consequences for many people. They aren't doing it out of malice, but out of ignorance or complacency.