If they were candid (instead of fearing some form of accountability), they would gleefully celebrate their actions and the end result.
You cannot shame them; they are impervious to it. The only "remorse" that they will exhibit is from any consequences that cause personal pain.
The people in history that were the functionaries that operated mass abattoirs came from somewhere in the then current societies. One of the most oft repetitive errors is the underestimation of capability.