From Black Mesa: Wiki

Overview

Black Mesa Research Facility is full of well prepared personnel that are used to team oriented working. This is no different in emergency situations, where both security and science teams help each other to reach a common goal. Nevertheless security personnel are prepared and enabled to react against any aggressive action against them or any fellow worker.

Friendly NPCs

• You can ask via the "use" key as many NPCs (scientist or security guard) to follow or quit you like in Half-Life. However you'll never have the opportunity to have more than three or four followers [1].
• Your weapon will be lowered while looking at friendly NPCs, but you can hurt/kill any NPC at any time, even while they are talking. [2]

Harming NPCs

• If you harm a security guard on purpose for a set amount of damage while no enemies are around, then he will shoot you back for a bit then stop. However, if you piss off the security guard too much or kill a friend in front of him, then he will become hostile and try to kill you.
• If you harm a scientist on purpose for a set amount of damage while no enemies are around, then he will run away from you for a bit then stop. The same goes if you kill a friendly in front of him.

End Game Allies

• None of these conditions apply if the friend is marked as an End Game Ally (if they die then you lose the game).

Example [3]:

"BMS_GameOver_Ally"
"ASSIGNMENT: TERMINATED\nSUBJECT: FREEMAN\nREASON: 
FAILURE TO PRESERVE MISSION-CRITICAL PERSONNEL"

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