As always, depends on your definitions.
If you use the most common definition in America, right-wing as referring to those defending the traditional American conservatives values as expressed in the Declaration of Independence and Constitution, then the answer is none. With the probable exception of the arguable genocide committed against the Indians in the 19th century.
If you use the European definition of right-wing, then there are a bunch of contenders.
Here's an excellent resource for body counts over the centuries.
https://www.hawaii.edu/powerkills/MURDER.HTM
I would argue that Hitler was indeed a right-wing mass murderer, as were his imitators in Croatia, Romania and elsewhere.
Nazism was a peculiar combination of generally leftist economics (never fully implemented, thought they might have been after the war, had they won) and Euro-type right-wing blood and soil social policies.
The reason I'd say his killings nevertheless qualify as right-wing is that they were almost all committed for reasons related to the Nazis' right-wing policies, not their left-wing ones.
I hasten to add there is very nearly zero correspondence between European style right-wingery, based on church and crown, blood and soil; and the American version.
In fact, I think the term right-wing doesn't properly apply to American conservatism, which upholds what is still the most radical revolution in human history. But it's a hopeless battle to try to correct the terminology at this point.
"I think the term right-wing doesn't properly apply to American conservatism"
I chafe when conservatives are lumped together with Hitler/ Mussolini.
Russell Kirk disagreed completely with that view.
Kirk's view is that the American Revolution was a profoundly conservative struggle. The American colonials were defending their rights as Englishmen against a king who was violating them repeatedly.