How close is too close to a kilonova?

Cataclysmic events happen in the universe all the time. Black hole mergers, supernovae, gamma-ray bursts, and a whole host of others. Most of them happen in distant galaxies, so they pose no threat to us. But there are a few that could affect life on Earth, and a couple could even pose an existential threat. One of these threats is known as a kilonova.