What about...
Players:
All Players
Conditions:
Current Player has exactly 50 kills score
All Players suffered exactly 1 deaths of Zergling
Actions:
Set kills score for Current Player: substract 50
Set deaths of zergling for All Players: substract 1
Preserve Trigger
There is a total of 84 used units&heroes in Starcraft, and 66 used buildings.
You probably won't use them all in any map you make using this system, and you can make seperate triggers for buildings because usually they don't respawn and don't move anywhere, what leaves about 50-60 copies of this trigger to make.
You'll also have to make replicas for every number of kills of every unit up to, let's say 8 (which is probably the most units that will usually get killed exactly at the same time), so 50 to 60 * 8 = 400 to 480.
Thus, using facts and a little thinking, I proved chu to be wrong

No, 1903821 triggers won't be needed, 500 triggers are more than enought.