The difference is a piece of paper (a "letter of marque and reprisal").
Are you just now getting your head around the fact that the legitimization of violence is a function of government? Damn, don't you folks teach civics in school?
Yep. That's the job.
Hell, if I could kill anybody I didn't like back when I was in harness, half the population of Oklahoma'd be dead.
And a truckload of sergeants and officers.