The news article at the top of this thread doesn't comment on any political affiliation. Why make the assumption?
Reports are saying he was contracted security for the news agency covering the protest.
"Photographs published by the Denver Post newspaper appear to show a confrontation during which the demonstrator strikes the security guard and sprays pepper spray at him before the security guard opens fire." https://www.bbc.com/news/world-us-canada-54497567
So a security guard protecting the news agency appears to have been attacked, and sprayed by a protester. The security guard retaliated to the attack.
Here's a photo of the victim and the security guard during the altercation. Mace visible in his right hand, guard appears to be gesturing past him, not sure what provoked it:
