Only You Can Save Mankind by Terry Pratchett
90'S PAPERBACK PRINTING [CORGI BOOKS]
It's just a game . . . isn't it? The alien spaceship is in his sights. His finger is on the Fire button. Johnny Maxwell is about to set the new high score on the computer game Only You Can Save Mankind. Suddenly, a message appears ... "We wish to talk. We surrender." But the aliens aren't supposed to surrender—they're supposed to die!