Half-Life-We've Got Hostiles

Gordon learns that the soldiers rumored to be coming on a rescue mission have already arrived. He attempts to join up with them, but walks into a room to witness a Marine killing a scientist. It is then that he discovers that the Hazardous Environment Combat Unit, which has taken control of Black Mesa, is not just killing the aliens, they are also targeting everyone connected to the project, including himself.
When he eventually reaches the surface, Gordon finds himself in an area totally under the control of the military and, worse, in the middle of an artillery bombardment; taking cover, he is forced back into the underground complex. However, it is revealed that the surviving members of the Lambda Team (in the Lambda Complex at the other end of the facility) may be able to close the dimensional rift. Therefore, Gordon begins the journey across Black Mesa to reach them.