MLK theory 1-
Hate-filled racist decided he didn't want MLK to live anymore.
MLK theory 2-
The FBI under J Edgar Hoover, the closet thing we've ever had to a dictator in the United States had been monitoring him. In that compete dismissal of the second amendment, Hoover became concerned that some of the people who offered their support of the civil rights movement and from whom MLK accepted support held anti-American views. In his thinking, because MLK was palling around with anti-American types and because he had become not only one of the most influential men in the back community but in America in general, he needed to be eliminated. Yes, that would be a violation of due process; life, liberty, and the pursuit of happiness; yada; yada but he reasoned he had been associated with these people and should have known where they were coming from.
However, MLK had an ally in the White House, LBJ. LBJ would not allow Hoover to touch him despite his sometimes out in left field associations. Then something happened to change that. After all LBJ had done to help the civil rights movement MLK made a speech condemning LBJ's Viet-Nam War policy and did so by essentially saying it was sinful. LBJ was so offended he gave into Hoover's repeated pressures and said have your way. One year to the date of the Viet-Nam War speech, MLK was killed after regular threats from Hoover.
Btw: although James Earl Ray served life in prison without a trial based on what some say was a coerced confession, a subsequent civil trial was held in Tennessee state court where the federal government was found liable in MLK's death.
Disclaimer: the above is mearly an unsubstantiated theory, is likely complete fiction and has no basis whatsoever in any factual events. This work of fiction should not be construed to imply any wrongdoing, either legally or morally has been committed by any persons living or dead.