The establishment saw King as a grave threat as his politics moved increasingly leftward towards a socialist perspective.
When MLK spoke out against the Vietnam War he effectivley signed his own death warrant. President Johnson is recorded as saying he felt personally betrayed when King did this. Johnson was crestfallen at the news that the anti-war movement had just received an enormous boost from the leader of the civil rights movement.
The assassination of MLK by the security services ties in with the FBI's shoot to kill campaign against the Black Panther leadership.
Hoover, as head of the FBI, saw King and the Black Panthers as dangerous threats to American capitalism which was undergoing a political, social and economic crisis by the late 1960s.