Easy peasy. Make two triggers, one will activate the win, the other will activate the loss.
For the loss;
Give your officer a name, you need this for later.
Open the trigger propeties and in the TYPE box, drop it down and select Loose at the bottom of the menu (yes it really is spelt like that in the game).
In the activation field of the trigger put;
!(Alive OfficerName)
This is the loss trigger done.
For the win;
Give your sniper a name, again, you need this for later.
Open the trigger propeties and in the TYPE box, drop it down and select END#1 near the middle of the menu.
In the activation field of the trigger put;
!(Alive SNIPERName)
And thats the win trigger done.
Now when the officer dies, you lose the game and when the sniper dies you win the game.