People were upset he was in uniform and the lyrics he screamed into the mic. 

Cops work security for concerts all the time. Officer Andrew Hicks was working security for the Vital Remains show. The band actually invited Officer Hicks on stage and he joined in the song “Dechristianize". This is where people got upset. Officer Hicks joined in the song and screamed the lyrics, "Let the killings begin!"

This incident was caught on film, that you can see above. It was sent to police chief, with apparently several complaints. Police Chief Cecil Smith said in a report that, “An incident of this nature erodes the thin fibers of trust which already exist between the community and the police, and it will not be tolerated within the Sanford Police Department."

Officer Andrew Hicks has now been let go by the police department. Before you start feeling too bad for him. He actually put in his resignation a month ago and his last day would have been today. This however, was the only incident Officer Hicks ever had in his six years on the force.