What if the French (or another country,like USSR o,why not,Italy) secret service had recognized Adolph Hitler potentially dangerous, and did kill from an agent in 1930?
Nazism would collapse?
You have the law of diminishing returns at work here. Let me explain. In
Chicago, you had North Side Gang and the South Side Gang. The North Side
Gang, in 1929, was headed by Bugs Moran. The South Side Gang was headed by Al Capone. By 1929, the North Side Gang had lost most of its
leadership in its ongoing war with the South Side Gang. Bugs Moran was
it. Then on February 14, 1929, came the St. Valentine's Day Massacre.
Capone brought in outside help. The killers were dressed as Chicago police
officers and detectives. To make it look all the more convincing, it was made to look like an ordinary arrest. Moran was still the leader of the North Side Gang, but the North Side Gang was a shadow of its former self.
The same, I think, would be true of the Nazi party. If Hitler was killed, lets say some time between 1921 and 1935, then his successor, whoever that
might be, will be slightly more competent, his successor, less competent,
and so on down the line.