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I'm looking for a story where it's being told from the "alien's" perspective about them observing Earth in its primitive times via telescopes from systems away.
They see us at our worst with war, violence and decide that we need to be purged for the good of the universe or something.
As the missiles are flying they keep observing and see us mature compassion and stuff but it's too late to stop the attack.
The attack hits and then they receive a message from Earth saying we're coming for you.