I’m far from an expert on the topic, but I’ve worked around the military for a while, and have had some conversations with some more knowledgeable people about this.
Basically, it’s any kind of military work that isn’t affiliated with a government. On the more benign side of things that would be stuff like private armed security, the French Foreign Legion, or working as a contractor for companies that do physical pen-testing for military installations.
It could also mean working as a contractor for a group like Blackwater, where you are engaging in some likely shady military operations where the government wants some sort of deniability, which (IMO) crosses the line fully into the malign.
Depends on the details. If their crime hurt someone innocent (or had a high likelihood to) and it was intentional, yes.
If I’m likely to go down with them if they get caught, yes.
Otherwise, I ain’t no snitch and even if I were I don’t know anything about nothing.