The Ur-Quan Kor-Ah have a right to defend themselves:

The black octopus space alien explained while ominous music played in the background:  "The Ur-Quan Kor-Ah have a right to defend themselves.  Potentially, all other life in the universe poses a threat to the Ur-Quan Kor-Ah, therefore we are engaging in a preemptive attack of self-defense."  As the Kor-Ah unleashed rings of fire, incinerating endless screaming civilians of all the galaxy's different races, it explained patiently:  "These sentient beings could one day surpass our technology, even though our buzzsaws do tremendous damage and have infinite range and have a homing function for anything that approaches too near them.  And what if those selfsame sentient beings then one day also spontaneously become hostile to us, despite the fact that we've done nothing at all to offend them?  It could happen.  The only way to be sure is to wipe them all out now."