I think u/MN-Jess ‘s explanation is correct, he kills a tribal police guy, who are dressed very similar to the security.

In the script here it looks like there were originally two people, one security contractor who said he could get up, and then one Bureau of Indian Affairs officer who got shot in the head by Curtis. I guess when they actually filmed it, they condensed it into one person and just made the tribal police guy say he was going to get up and then get killed

Ah okay thank you, this makes a lot more sense now, I can even see they have different patches on their shoulders. I wonder why the tribal police guy spoke up when he could’ve played dead.