The main historical significance of 9/11 is that it served as a pretext for a stepwise escalation in US fascist narratives & policies, as part of the right's decades-long project to remake the country as a blood-and-soil homeland.
It was one more piece of the permission structure.
Basically post-WWII US history has been the long process of the US fascist movement, which had been decisively suppressed & rejected as part of the global fight for democracy, reconstituting itself & slowly but surely bending the country back to its will.
They used 9/11 as an excuse to accelerate, sure, but they use every major historical event that way. Carter's hostage crisis, Afghanistan withdrawal, the election of a black president, take your pick -- they're always pushing, pushing, pushing in the same direction, so any event is an excuse.
So, folks, the fact that they're using Kirk's killing as a pretext to accelerate their fascist rhetoric and policies is the least surprising thing to happen in history. There's never been a development more predictable.
They will blame the left for every step they take toward fascism. "You were mean to Robert Bork! You called Mitt Romney names! You tried to make us be nice to trans people! Some nutbag tried to kill Donald Trump!"
You are a *fool* if you take any of that at face value. A dupe. A mark.
They want to move in the direction of fascism. They built a whole movement for it, spent decades & decades working at it, have devoted billions of dollars and thousands of hours of work to it. They don't need excuses or pretexts!