I'm looking for a short story about how scientist aliens look at earth from afar and see a violent race bent only on killing each other. The aliens decide to destroy earth as to eradicate the threat to the galaxy/universe.

The weapon (a laser) takes some time (hundreds of years?) to reach Earth, meanwhile, the aliens see that humanity has evolved and has become peaceful towards itself and start to regret their impending doom.

After Earth is destroyed, most(?) of the planets of our system light up in mourning and a human fleet is sent towards the source of the laser.

The last thing I remember is a quote that is sent to the aliens at the end stating "We see you" or something along those lines.