Uncle Jimbo's Bug Huntin' Range
The ancestors of the Just were a minor species of no particular note, spreading by slow-ships to about a dozen worlds. They raised their hand against Humanity, and Humanity dealt with them harshly, as was their way.
By a historical chance or unguessed reserve of strength, this species united to a single purpose, to avenge the arrogance of the dominant race. They skirmished and evaded through a thousand years of conflict, defeated, broke, and hunted out every vestige of Humanity.
When their task was complete, the Just were changed, utterly. No longer able to understand anything else but war, and with no-one left worthy to fight, they departed to give justice elsewhere. But they are not forgotten. Those who seek to raise themselves as Humanity once did, take warning, and beware the vengeance of the Just.
That's not to say that Just cyborg dreadnoughts will pop up from a wormhole whenever a species fires a nuke or builds a starship. They're more like Hell and the Devil to mediaeval Christianity - a consequence so fearful that no-one really expects it to appear for mere earthly sins.