Deadlands 2: Trapped (2008)

"In one night, One city will fight against an enemy that never stops, shows no emotion, no remorse, and is already... DEAD!"

The United States Government has developed a highly effective nerve gas that is unleashed upon the residents of a small Maryland City. During the top secret exercise the infected citizens begin to convert each resident into a sort of living zombie, until the remaining town residents, 6 total strangers, are trapped inside a movie theater cineplex.