Magnificent Baddies who have attempted or, in the worst of cases, succeeded in commiting Genocide, the crime of eradicating an entire ethnicity, species, culture, religion, etc.

Genocide is usually carried out through the mass murdering of the targeted group, although, in the case of Ethnocide cultural/religious Genocide can also be done by eliminating all records of said culture and converting all members until no-one follows that culture anymore (in Sci-Fi settings, this can often be done through Mass Brainwashing).

Genocide is easily one of the most heinous crimes one could ever commit, if not the absolute worst, and most Genocidal villains will be far too deplorable to qualify as MB, especially due to Genocide and Xenophobia almost always overlapping. To some much as be a possible MB candidate, the Genocidal villain's agenda must be born of utilitarian or even delusional, extremistic, or Anti-Villainous reasons and not from outright bigotry towards the group and the villain will often need a lot of sympathetic qualities to balance things out and prevent them from being too heinous, or even grow out of their genocidal ideals and become Wholly Grown.

Alternatively, MBs from this category could avoid being too heinous due to living in historical or cultural contextes in which Genocide isn't nearly as frowned upon as it is for us (i.e. Ulysses).