OnPCKillEvent will only be executed when a player will kill another player so to achieve a global monster drop use OnNPCKillEvent
So the script should look like this:
OnNPCKillEvent:
if( rand(1000) < 10 ) // Chance: 10 = 1%, 100 = 10%, etc.
getitem .item[ rand( .item_size ) ],1;
end;
OnInit:
setarray .item[0],<item id1>,<item id2>,<item id3>,<more>;
.item_size = getarraysize( .item );
end;