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I read a short story online a year or so ago. Told from an alien's point of view, they discover Earth and are so horrified by mankind's warlike nature they fire a weapon at Earth that will take a few centuries to hit us.
They then watch as we evolve beyond the middle ages and realize we're not that warlike. The scientists involved in creating the weapon kill themselves by 'Sealing their spiracles' I seem to remember.
It ends with humanity developing fast enough to stop the weapon, then sending a message to the aliens along the lines of "big mistake".