Right - you don't need a trigger for this.
Double-click on your civilian unit, and paste this into his "Init" box:
this addEventHandler ["killed", {_this execVM "whoKilledHim.sqf";}];
Now, in your mission directory, make a file called "whoKilledHim.sqf", and put this in it:
_sideKiller = side (_this select 1);
if(_sideKiller==west) then { endMission "END1"; };
if(_sideKiller==east) then { endMission "END2"; };
This will cause the mission to end if he is killed by one of those two sides. Also in your mission directory, make a file called "Briefing.html", and put this in it:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
</head>
<body bgcolor="#FFFFFF">
<! -----DEBRIEFING----->
<hr>
<br>
<h2><a name="Debriefing:End1">West Wins!</a></h2>
<br>
<p>
West killed the civilian!
</p>
<br>
<hr>
<br>
<h2><a name="Debriefing:End2">East Wins!</a></h2>
<br>
<p>
East killed the civilian!
</p>
<br>
<! -----DEBRIEFING END----->
</body>
</html>
Hope that helps