Right, but if you ask J Edgar Hoover supporters, MLK was an adulterer and a Socialist / Communist agitating the American populace (during a time when mass protests and riots were spreading like wildfire). The thing is, there is some truth in those claims.

Also, American society in the 1960s was not nearly as supportive of MLK Jr when he was alive as much as they are now that he was martyred. You and I are looking back and projecting our modern values on people who were still largely racist in a time when racism was both a de facto and de jure truth.

That is to say that lots of America would likely have supported the effect of the blackmail letter, if not the method itself. It was a different time. The frequent threat of the USSR, encroaching Communism, and the specter of nuclear war was suffocating and the "Law and Order" marketing of Nixon and J Edgar Hoover were appealing to many people, even if they suppressed the freedoms of many.