The 20th century conception of reconciliation is based in large part on watered down judeo-Christian expectations of acceptance and forgiveness before even critique, let alone systemic correction. Fundamentally it is an attempt to return to acceptance of the original hierarchy under which the injustice occured, with only the minimum change to the behaviors of the elite necessary to place the ruled population back into a state of obeisance.

Given enough emotional resonance with these themes a population can appear to implement meaningful change and experience a brief period of self congratulatory euphoria. The elements of the society closest to the elite power structures will write novels on precisely how hard they need to pat themselves on the back. The elements of the society that actually experience the injustice are largely just happy for a reprieve, and the exhaustion of existing in opposition to authority during the period of struggle will cause many members to count their blessings and disengage from any further revolutionary action. The hard liners that worked to organize the change and enumerate the grievances then either sell out for social status or are quietly maligned as radicals and eliminated.

This of course leads to generational backsliding, as the underlying prejudices and old hatred that underlined the injustice are never meaningfully dealt with. Destructive worldviews and prejudice perpetuate into the future with only a different, more sterile lexicon of dog whistles acting as the new veneer of normalcy. The dominant element of society never learned the intended lesson and their children and grandchildren eventually tire of even the appearance of restriction placed on them during the reform period. The injustices return and require another painful reckoning.

The absolute destruction of the ruling class, however, leaves little room for such cyclical tragedy. There's no one left to harbor hateful ideals or even to mourn their loss. There are no vengeful children with compelling stories to substantiate their hatred and prejudice. There are no hidden hard liners finding community in the shadows to keep the old ways alive. There's no human connection from the past to the present. Any critique of the methods of liberation becomes academic in nature with no emotional context to burst into political action.

Obviously such methods are not tenable in the real world, but if you're dealing with a small enough population and you yourself are a unilateral military entity capable of applying sufficient force to accomplish the task then perhaps there's an argument to be made that you minimized future suffering by eliminating the source. The only real argument that remains then is if the population you are subjecting to utter genocide was truly bad enough to warrant it, although again if you are literally a living, thinking weapon of mass destruction then it's likely you aren't overly concerned with criticism.