There are many theories that discuss the assassination of Dr. King. Some theories suggest that James Earl Ray killed Dr. King to gain a name for himself. He was a high school drop out and petty thief yet he left a lot of evidence such as a gun with fingerprints and a personal radio. This leads to various conspiracy theories.
Many people, including Jesse Jackson and Dr. Kings family, believe that it could have been a conspiracy set in place by the government. Dexter King even met with James Earl Ray at one time and stated that he believed that he did not shoot Dr. King. James Earl Ray himself later said that he in fact did not kill Dr. King and asked for retrial but his request was denied.
If he did kill Dr. King, I would assume that it was due to racial discrimination. I would assume that he did not want blacks to have equal rights. These are just assumptions.
There are few verifiable true sources that can pinpoint "A" motif to conspire to kill MLK. Somethings are known commonly however such as a) Ray was indeed racially biased against black people b) Ray had been a criminal prior to the killing c) he did play an audience and seemed to enjoy the spotlight when it was on him, so he may have just been a sorry man who wanted to be notorious since he would never be famous.
However, nothing is 100% clear because MLK was a wanted man. Period. He had upset Lyndon B Johnson with his anti War manifestations. He even had BOTH the Black Panthers and the KKK mad because he was a unitarian and they were separatists. The horrible reign of the FBI wanted him removed because MLK was causing too much rumble with the civil rights movement, and one must remember that this was the first African American man ever to be held in such international and national esteem and fervor: He literally moved a mountain.
So, this is why it seems almost illogical that a virtual "nobody" like James Earl Ray would be THE man to finally finish the uber shaker King. Seems, however, that he was more than willing to play the scapegoat. What else would he have to lose?
No one really knows why James Earl Ray decided to kill Martin Luther King, Jr. He did not say why he did it and he later denied that he had actually done it.
The most usual argument is that Ray killed King because he wanted to be famous. People say that he really did not have a huge political motive or anything. However, other people do not believe that that could possibly have been enough of a motive for him to go to all the trouble he went to. They also argue that it looks as if he had support from someone with money.
I'm including a link to various conspiracy theories... The CBS link has fewer theories, but is a more credible source.