Run to the Stars by Michael Scott Rohan
The protagonists realize that the "BC" world government is engaged in a top secret conspiracy. Half the plot is their efforts to discover the truth.
As it turns out, government scientists have received radio messages from an alien species. The government decides that the best course of action is an act of genocide to kill all the aliens. They design a relativistic missile large enough to pulverize the alien planet. All in secret, because many of the citizens of Earth's world government would be quite angry if they knew what was being planned.
The protagonists are shocked. But then they have a fierce internal debate. Some of the protagonists think this genocide might actually be a good idea.
However, other protagonists point out this will mean Earth will have to hide from the stars for the rest of eternity. The relativistic missile will leave a trail pointing straight back to Earth, evidence of Earth's crime of genocide. If Earth sends out starships to explore the universe, and stumble over other species of aliens, it will be trivial for the aliens to discover Earth's crime. The aliens will then try to destroy Earth as punishment for Earth's crime.
