I have a new vote for probably the most consequential assassination in world history, and it isn't Franz Ferdinand.
Patrice Lumumba was a Congolese freedom agitator during the time of the Belgian Congo, and had one of the greatest speeches in world history during the independence ceremony of his country.
Of course, Belgium was pissed that they lost all of that land, so they Jumpstarted the Congo Crisis by supplying guns to rebel tribes, and when Lumumba begged the UN for aid, they gave little to no help. Lumumba, an avid Socialist, then turned to the Soviet Union for help instead.
The CIA, under directions from Eisenhower, had Lumumba ousted, and practically given over to the rebels where he would be executed on January 17, 1961.
As a result, the CIA became more bold over the next decades, couping more countries and democratic regimes out of their incredible paranoia of Communism.
Congo's coup would set up the dictatorship of Zaire until 1997, but the people of Congo still suffer to this day, with constant civil wars, disease, poverty, etc.
Long story short, Leopold II should be demonized at every single step.