I debated for a long time if this question would be better suited in /r/HistoryWhatIf, however I'm specifically asking what exactly in the history of America (including large parts South America) or the States prevented the existence of a dictatorship or regime that lead to mass murders and/or genocide.
I quickly drew a map on my phone with the reqions affected by a dictatorship that involved the death of > 20'000 people based on this source (Worst genocides in the 20th and 21th century):
Most of the events, genocides or dictatorships took place in the Balkan, South East Asian, Middle East or African regions, while I didn't found anything on that scale for America.
So, what exactly prevents those regions from falling under a regime etc.? For US: The constitution? Or the fact that many regions are not natively inhabitated (except by the natives of course), but conquered by settlers?
The majority of South America did go through dictatorships in the 20th century though, Chile under Pinochet, Argentina under the Junta, Brazil had a military government etc. In Central American countries you also had outright dictatorships (i.e Guatemala the years immediately after the coup, Panama under Noriega), quasi-one party states (i.e Mexico under the PRI).
I like the map you made but you might be missing some wars, the Guatemalan Civil War for instance was brutal and led to the death of over 100,000 people, El-Salvador's civil war also killed tens of thousands.
Actually come to think of it, I can't really think of a single Latin American country which had stable democratic politics 1950-1990.
I'm curious as to why you don't think that the conquest of the American west constitutes a "genocide" of the Native Americans.
Or, for that, matter why African slavery would not count if you're also including "mass murders".