Communists killed the most, followed by fascists.
At the insistence of Josef Stalin, the Genocide Convention applies to some mass murders by government (e.g., targeting racial, ethnic, or religious groups) but not to others (e.g., targeting victims for class warfare or ideological reasons). Hence, prof. Rummel coined the term “democide” to describe noncombat mass murders for any reason.
https://reason.com/volokh/2022/11/09/data-on-mass-murder-by-government-in-the-20th-century/