I think the most endemic and destructive cultural phenomenon of all time has to be the tendency to look for ONE answer to all of the world’s problems.
Whether it be ONE religion, ONE way of governance, ONE empire, ONE rule, ONE way of thought… The tendency to believe that putting all things in one basket will cause all problems to end is by far the most dangerous cultural phenomenon ever.
People out to conquer other nations so that all lands fall under one nation and people out to convert all people into their one true religion are tools of such totalitarian thought.
Such behaviour has done immeasurable harm to the diverse nature of the world and continues to do so. The world was made to be diverse and this is not a theory. A cursory glance at nature confirms it.