I've been looking for about an hour or so now and I can't find it.
It's a lua file that changes the AI relationships to be based on your player model. So If you have a combine player model, Zombies and Rebels will kill you. If you have a Zombie, Rebels and Combine will kill you. I don't remember what it's called and I've been googling this for a while now. :/
Blah, just found it. "Player Factions 3.0" Dunno why I didn't think of the word faction.
