The Poisonous Song of the Skies
We saw it by accident. It wasn't the sort of extraterrestrial signal we had been looking for, nor the type of star we though it would come from.
Yet it was a clear signal after its own fashion, and even clearer when we combed our observation archives with the help of urgency and hindsight. One of the star's planets -- not, by far, one of the larger ones -- had experienced a frantic rise in atmospheric carbon, followed by a spike of exotic elements not generally found outside laboratories. It was intelligence, if not a particularly smart one. Effective, though. Finer analysis of space-based recordings showed there had been faint electromagnetic signals coming from the system.
The revolution in astrobiology triggered by our discovery was thus shadowed by the socio-philosophical one. But that's not what worries me.
We had never conceived of this kind of weapon until we reverse-engineered these observations. Now we're building them, and I know of only one civilization that did, and they didn't end well. Maybe they'd have been happy to think the light from their demise could be a dangerous lesson but nonetheless one, but I suspect they were the kind of species that would've felt a dark, petty pride in having cursed us to reenact their suicide.
If we do, I hope it ends with us - but I no longer watch the sky at night, or wonder at its quietness.