...or you could completely forget the script and just use a trigger
TriggerRadius: 0,0
Condition: none
Type: end#
Condition Field: "not (alive _x)" count [
c1, c2, c3, c4... ] > 0
That will end the mission 'immediately' if a civvie is killed.
You may want to use a variable, as Coldsteel suggested to have an end cutscene or a message about killing a civilian. Up to you I guess