Clausewitz did not say talk about THE decisive battle. He talked about the need for Decisive battles that gained you benefit in terms of strategy.
Well, here we have a dialectic/translation problem. I was not talking about THE decisive battle but using the as in French where “the” would mean “a”.
Also, I believe this could be debated but I did not read CvC in German.
But I have to admit that benefits from Waterloo were longer term than Austerlitz on the political side. But the benefits from Austerlitz were larger at its time. (more countries involved, larger impact in European powers…).

Just to add some spices to the discussion. Definition of a decisive battle is quite loose with time. I’ll take the example of La Marne in 1914. It was merely a strategic decisive battle but was a tactical decisive battle as it was more a smart use of modern logistic technology and more or less did fix the front. But this was not the attempt or aim. (The objective was to protect Paris so the old dogma I have your capital = I won, would not happen).
But still it was a decisive battle.

Closer from us, I can see where such statement that “mass killings benefits” could come from.
I will take the example of last Israel operations. (No critics, no offense, no judgment, please. Take it as an intellectual exercise).
The 2006 summer war was a defeat and Israel needed to reaffirm its military supremacy. So they did conduct Castle Lead.
I have no idea of the ratio of population killed among Palestinian. But IDF maintained a 1/10 ratio if you melt civilian+armed population for the Palestinian. (Roughly 1 IDF for 10 Palestinian). And, as you said, Israel gained some time.

Was it a decisive battle? Personally I would not go that way. The Rand studies about Israel cycle of violence have shown that basically Israel is buying 11 month of peace between each round of terror/war.
On a regional scale? Well, that can be debate also since Iran has shown capacity to produce continental range weapons.
On the hostage scene? Yes most probably.

But does that really balance the bad image that Israel did built during the operation? Did that worst the lost of credit among the international opinion (even in West)? That is not that clear. Just for this I would say that the statement of killing plenty, even combatants, would be somehow fault. It just shows a theoretical understanding of war. Also, CvC did not write about stabilization and stabilization was much easier at his time. Well may be not in Spain.

In war among the people and with the increase of education of the population (civilian + military + international opinion) such assumption based on terror is almost an immediate political loss. So the benefits from an operation or war that kills many civilian (the figure should not be count in %) has more disadvantages than advantages.

Or the guy is just crazy.