Yes. Give each of the civilians a killed event handler that increments a counter then check the counter.
Create a file called init.sqs
in it put
CiviDead = 0
then if you don't have too many civis put the following in the init field of each
this AddEventHandler [{killed},{if ((_this select 0) == player) then {CiviDead = CiviDead + 1}}]
Create a trigger that has the following condition line:
CiviDead >= 15
and the following activation line:
hint format["You fool you have killed %1 civilians",CiviDead];"0" objStatus"FAILED"
Assuming object 0 is the one you are dealing with.
If you have a lot of civials then set up a trigger that covers the map and detects civis - bear in mind that empty vehicles as seen as civilians so you need to ensure you only detect civis of the two legged variety. Then use the trigger to give each the event handler