So the holocaust is bad because of the intent rather than the outcome. Said intent is to eradicate a people. This seems similar to the Roman eradication of Northern European culture/religion. Racially they were radically changed as well. Example: the Roman invasion of Gaul. One third of the population was destroyed, the gauls as a people never existed again. Yet it's a minor historical foot note.
The Holocaust is evil, not just bad, because of the outcome. The intent, and the outcome, is what makes the Holocaust different to the deaths of other civilians during WW2.
Other peoples have been butchered in invasions, slavery, war, etc, but these acts were done as part of some strategic plan eg to gain or maintain power, territory, etc, or incidentally as collateral damage, or for some economic or utilitarian reason.
The Holocaust, on the other hand, had no strategic war interest, no military or even political gain, but was done purely for the sake of exterminating one particular ethnicity, ie the Jewish nation.