EDIT: Something weird is happening. On my end I've observed OpFor shooting other OpFor units who were affiliated with the Civilian faction after doing what I explained below. I'm trying to figure out how and why now, because I haven't been able to replicate it outside of the mission I've been busy with for the past few weeks.
So I've had a dilemma. I'm working on a game mode where I want there to be a four-way conflict between four hostile armies. Problem is, ArmA only has three military factions; BlueFOR, OpFOR, and the Resistance/Independents.
Previously, the popular notion is the military will NEVER shoot civilians. This is true. They won't shoot civilian
units. However... there's a way around this.
First thing which you probably already knew, is you can make civilians hostile toward other factions. You can do this by adding the following to your Init.sqf
Civilian setFriend [EAST, 0];
Civilian setFriend [WEST, 0];
Civilian setFriend [RESISTANCE, 0];
However, if you do the opposite...
EAST setFriend [Civilian, 0];
WEST setFriend [Civilian, 0];
Resistance setFriend [Civilian, 0];
... the main factions still won't shoot back at the civilians even as they slaughter their men. Not
yet anyway. Remember what I said, they won't shoot at civilian
units. However, the same is not true for the civilian
faction.

Remember to set the civilian leader to have no chance of spawning.
Military units won't fire on civilian
units but they will fire on other military units. I forget what inspired me to try it, but I plopped down some Independent units to see what would happen.
Low and behold,
the Independents started shooting the 'civilian' faction!!
Hopefully this will help people make a hostile "civilian" faction.