To the moooooon

I just want to go back in history and ask at what point people would’ve considered it appropriate to use political violence against someone like Hitler, or was it never okay?

Depends on the country

In Germany:

Communists and Anarchists: Around 1920, when the SA was founded

Social democrats: Depends on the „type“ of social democrat. The Reichsbanner Schwarz-Rot-Gold, the social democratic militia, expected their leaders to start a coup in 1933

Conservatives: 1939, when the war started (at least those, that still were conservatives and didn‘t join the Nazis)

Monarchists: 1942, when it became visible, that Hitler couldn‘t win the war (at least those, that still were monarchists and didn‘t join the Nazis)