As per this extensive recap of the movie, that man clearly has been seen before in the movie:
Nic goes into the nearby bar looking for a phone. He finds one but it doesn’t work. He then looks up to the television where the news has classified Haley and him as biohazard threats that should be considered dangerous. Nic looks out where he left Haley and see her enter a truck that is leaving (despite his directions to stay put). Nic races after the truck and tries to get it to stop but the driver pushes him away. Then, Nic, getting in sync with his new legs begins to run very fast and catch up with the truck. The driver, James (Robert Longstreet) reaches for his gun but finds it missing. He looks to the passenger seat and sees Haley there pointing the gun at his head. He immediately stops the truck. Nic and Haley steal it and drive off.
The scene you describe:
We see Damon and his team lock down a building. Nic and Haley hear something while they are sleeping outside and go to hide. We are led to believe Damon has found them but in actuality, they have quarantined James in his home. Damon goes to speak to him and is calm despite James pointing a shotgun at him. James talks about his family being there with him (which they are not). Damon nods along and tells James he was strong. James tries to shoot Damon but his shotgun doesn’t have any shells to his shock. Damon tells him it is okay and shoots him in the chest, killing him. He then covers James’ body with a blanket and pats his shoulder.
The horrifying realization hits Nic; all the “government officials” including Damon are aliens. Everyone else are human abduction victims being slowly driven insane by failing to condition to the alien technology (that’s why James and Mirabelle went mad after coming into contact with Haley and him).