I was playing the reverend swanson mission and i had just knocked out the rancher who was grappling swanson so i knock him out and next thing i know there is a witness. and since its literally pretty much the middle of no where i just shot him and a lawman just came out of nowhere and shot me then i failed the mission.
I then retried the mission a few times to see if it was a bug but nope, each time i killed him a lawman would spawn right next to him and shoot me, and i would fail the mission and it would say "the law was alerted"