Probably depends on who you talk to since yes, many Americans were horrified, but it only directly affected a handful of families overall whose lives were changed forever. The general consensus is that it was the end of America's "innocence" in terms of what was considered a peaceful time. It was terrorism on a level unexpected to the average American, and it's never been the same again. At least two generations were negatively affected in the economic fallout. But for a time, Americans came together because we all were targets as infidels.
Charlie Kirk was killed by an American groomed by a dangerous ideology, and there's groups of other Americans who were cheering it on. It's shocking and horrifying because now you, as a white straight man and other Christians in general, are being targeted by other Americans who hate your guts. It's not even one group as much as people are going for a "right vs. left" approach, it's as simple as "good vs. evil" and there are legitimately evil Americans among us right now.
9/11 was foreign terrorists hating Americans. Kirk's assassination is Americans hating other Americans with a different opinion and belief system. It's much more personal and scary as a result.