I’m rather surprised by the online OED’s definitions of assassinate and assassination.
It defines assassin as:
1. lit. A hashish-eater. Hist. (in pl.) Certain Muslim fanatics in the time of the Crusades, who were sent forth by their sheikh, the ‘Old Man of the Mountains,’ to murder the Christian leaders.
2. Hence: One who undertakes to put another to death by treacherous violence. The term retains so much of its original application as to be used chiefly of the murderer of a public personage, who is generally hired or devoted to the deed, and aims purely at the death of his victim.
It defines assassinate as:
1. trans & absol. To kill by treacherous violence.
2. trans. To endeavour to kill by treacherous violence; to attack by an assassin. Obs.
3. fig. To destroy or wound by treachery; to ‘stab’ reputation, etc.
It defines assassination as:
a. The action of assassinating; the taking the life of any one by treacherous violence, esp. by a hired emissary, or one who has taken upon him to execute the deed.
b. fig. Cf. ‘killing’.
I’m sure that most people asked to define assassination would say something along the lines of “murder for political or ideological motives”, and would agree that the motive is a defining characteristic - one does not ‘assassinate’ a business rival or an unfaithful husband. And although one can certainly hire (or be) a professional assassin, if one takes it upon oneself to kill one’s political or ideological opponent that is also assassination, surely. This isn’t a new sense: it’s how I have understood the word for my entire life (D.O.B. London 1956), and I find it very odd that motive isn’t mentioned in the definition at all, and only indirectly alluded to in description of assassin as “the murderer of a public personage”.