Thinking too much domestically in my last post, I missed the obvious, no-brainer #1 choice of all time:
Archduke Franz Ferdinand of Austria-Hungary, whose assassination in 1914 set off WWI which destroyed the old order and begat communism, Naziizm, and the resulting spiritual death of the West. (and the deaths in WW1, WW2, and under communist tyranny, of some 200+ MILLION people, many of them the ‘best and brightest’ that their nations had to offer).
At least he lived long enough to sign my grandparents’ exit visas from the Austro-Hungarian Empire.