Perhaps after detecting intelligent life and reporting it, perhaps it attached itself to the largest convenient asteroid/comet, and altered it's orbit just so to stunt human development just enough to allow for a more complete extermination ship to be dispatched.

Like a galactic immune system - suicide berserkers designed to signal the human infection, and then suicide themselves to slow the disease evolution.

The galaxy is many billions of years old. I would not rule out the development of such a system.