Why did James Earl Ray kill Dr.Martin Luther King Jr.?

Answer:

James Earl Ray had attended a 'George Wallace presidential campaign rally' the day before he killed Martin Luther King Jr. George Wallace was extremely racist and segregationist. He had run for president on the Democratic ticket but could not become their candidate, so he ran as an Independent. George Wallace was Governor when he stood in the way of a public school as black students tried to enter after segregation was abolished. James Earl Ray was inspired by the Wallace's racist philosophy and rhetoric and decided to kill Martin Luther King Jr.
As an aside, though the Democratic party once included segregationist Dixiecrats, most (but not all) of them abandoned the party during the civil rights era to become Independents or Republicans. Good example: Strom Thurmond.