I understand the point here, and it is indeed an important one. It is of course also especially pertinent to the Holocaust, where I happen to agree that an argument which represents the Holocaust as morally distinct from other 20th Century genocides is problematic.
What is the difference between manslaughter and first-degree murder? Surely that is a moral distinction that leads us to impose different penalties on the person responsible, where the moral distinction rests on the motives or reasons that the act was committed, not on the fact that an innocent person has suffered a wrongful death.
Whether I carelessly run someone over with my car in such a way that I am culpable in their death, or methodically plan to kill someone, the person is still dead. By Volokh’s argument, there is no moral difference: wrongful death is wrongful death.
There really are two separate moral questions here (and of course, as Volokh observes, an entirely different class of substantive questions about causality, consequences and so on). The first is the fundamental deprivation of the rights of an individual through wrongful death. This is flatly and evenly morally wrong regardless of the circumstances of death. Then there’s the issue of why someone killed someone else, and that is absolutely a moral problem, and a variable one.
There is, for example, a moral distinction between civilian deaths incurred by accident in warfare and acts of war which set out deliberately, programmatically to kill non-combatants. If all that mattered morally was wrongful death, that distinction would have no meaning, because the people who die by accident would not have died had the use of military force not occurred, and could not be distinguished from those whom military action was meant to kill.
This is the ground on which those who distinguish the Holocaust as a morally unique wrong stake their case, that the combination of a deliberate and bureaucratic program of mass murder with an ethnic or racial motivation is different than mass murder by Stalin’s regime or other statist authoritarians, where death was more typically the result of indifference to the consequences of confinement to prison camps or resulted from almost random executions of whole communities. Here I agree with Volokh: the moral distinction is minimal, and we learn and understand more by placing the Holocaust within a continuum of 20th Century events that include Leopold II’s regime in the Congo, the USSR under Stalin, Cambodia under Pol Pot and the 1994 genocide in Rwanda. But the moral continuum in this case includes the motives, means and consequences: Volokh is wrong to insist that the only moral issue is the last of those three.