A government is responsible for itself and it's people. It is not responsible for the well being of the world at large.

You're missing all the treaties the U.S. has ratified that do impose obligations to the rest of the world on it. But even if you ignore all of that, a state that has no qualms about mass murder outside of its borders is a dangerous, violent state that should be destroyed.

This is also a silly response to "what would you have done?". It's not about what states historically have done, it's about what the U.S. could have done that would have made it worth supporting. In the immediate aftermath of WWII the U.S. had military, political, economic, and social influence unparalleled before or since. It could have actually remade the world order, or at least tried, but it instead chose to continue imperialism with itself in the driver's seat. It was in no way forced to do this, and its decision is worth criticism.

US citizens generally approved of their government's actions after the war

U.S. citizens generally approved of the genocide of indigenous Americans, too. Just like democracy does not extend to voting to kill someone, it does not extend to committing genocide (which the U.S. supported and directly aided throughout the Cold War) and other war crimes, no matter how popular they are.

Just ask Japan - the US could have annexed the entire country and enslaved everyone.

"I could have killed my wife, but I just broke her arm! She should be thankful."