I'm trying to remember the name of a story I read some 20 years ago.
In the story a powerful alien civilisation is confronted by a strange enemy: a 2-dimensional rectangular shape that destroys their ships. The "shape" is actually a projection of a computer screen, and the "enemy" is a human that plays a computer game. The aliens figure out how to deal with the situation and capture the human, who is then judged for the many deaths he had inadvertently caused. While he is waiting for the judgement in a small capsule in space, his friend on Earth logs into the game, sees the capsule, and destroys it.