Dr King was considered a political figure. It wasn't religious, otherwise other Baptist pastors would also have been targeted. It was his active role in the African-American civil rights movement that got him killed.
You have to be a prominent political figure. Not necessarily a national leader, otherwise the murders of Dr Martin Luther King, Robert F Kennedy and Mohandas K "Mahatma" Gandhi wouldn't have been considered assassinations, but all were actively involved in politics.
Hi Element~ In my humble opinion, a murder is done with passion. An assassination is done as a hit job.
It really depends on the feeling, or lack there of, the person has who is pulling the trigger.
To some, John Lennon's murder was as big as John F. Kennedy's assassination.
According to Wikipedia, the definition of assassination vs murder is: Assassination is the murder of a prominent person, often a political leader or ruler, usually for political reasons or payment.*
To me, they both were very powerful people who spread words we will all remember. Why one is defined one way and the other another is about where you place the power of the people and the media.
Benedict Arnold 