See this.

"Assassination is the targeted killing of an individual who is in a high-profile position. An added distinction between assassination and other forms of killing is that the assassin has an ideological or political motivation, ..."

Use your Google.com search with an expression "define:assassination" for more.

Originally only heads of state got "assassinated". But the application of the term seems to have been creeping down the hierachy of prominance. Any judge in Irac killed by the insurgents is being called an assassination. Even rival heads of organized crime are said to be "assassinated".

Prejudice affects all abstraction by anyone - including when a person consciously tries to compensate, perhaps swinging too far in the opposite direction.

If you think a suicide bomber is not politically or ideologically motivated, your perception is wildly different than my abstraction. The personal motive and the organizational motive may differ sharply; but an act of terror, so branded, is previously defined as ideologically or politically motivated.

Ben, your question about the historic evolution of the word seems to try to apply a strict division to a fuzzy evaluation that varies with individual to individual. I might suggest that a lexicographer might abstract and trend and arbitrarily choose a point to mark the distinction. In this case the "map" very significantly reflects the map maker. Your question can not be answered as an "absolute", because it asks for what becomes a concensus opinion of map-makers - users of the term.

Your question Did calling MLKJr's killing "an assassination" mark a turning point in the use of the word? implicitly presumes the "exsitence" of a "turning point", capable of being marked by a map-maker, so it paraphrases to " where 'is' the turning point".

Like a glass of water, where "is" (Milton: "is" of existence.) the division between partly full and partly empty?

Assassins may have done the killings, but Wikipedia also states: The earliest known literary use of ... "assassination" is in The Tragedy of Macbeth by William Shakespeare (1605).[6][7].

To report on the etymology of the word 'assassin' and assume it applies to the derived word "assassinate" or "assassination" is to confuse the abstraction process - a "straw man" argument. The assassins carried out the murders, but the word 'assassination' did not get recorded and presumably was not in use much before Shakespeare's time.

(One semantic quibble deserves another.)

No, my use of quotes around the word make the context opague rather than transparent. Without the quotes the word transparently refers to the act; but with the quotes the context is no longer transparent; it refers to what was said, in this case the word; and I was referring to the use of the word applied to the act. You had the relationship between quotes and transparency backwards. Who in Shakespere was said to be "assassinated"? Henry VI. Was he not a head of state? According to the time-binding record, the first use of the word 'assassinated' or 'assassination' referred to the killing of a head of state, albeit in a work of fiction. Prior to that many assassins killed many a person for many ideological reasons and just for money, but the act had not yet been labeled "assassination".