One instance of political assassination in Shakespeare's play is the murder of Julius Caesar by the different members of the conpiracy against him, namely Cassius, Brutus, Casca... They feared the Republic would be overthrown and Julius Caesar would become king if they didn't hamper Mark Anthony's efforts to persuade J. Caesar to be crowned. The assassination somehow conjures up the image of a sacrifice: "a dish fit for the gods."
This is obviously quite different from Macbeth's wish to ascend the throne. We musn't forget that even if Duncan, the old king had died, His elder son, Malcolm would have had a legitimate right to lay claim to the throne as Duncan chose Malcolm to succeed him after his death. The murder of the old king is the representation of Macbeth's illusory search for power. The "golden ring" that symbolizes the crown is germane to the golden fleece of the Argonauts but here the quest is a pervert one. Macbeth's reign is hardly dealt with in the play but it is a long uninterrupted chain of murders which have nothing to do with a cause or an ideal. His personal ambitions must be fulfilled. Murder was the path to kingship and murder is the only way for Macbeth to stay a king. At the end, Macbeth's reign proves to have been a mere "interregnum" and Macbeth a mock king.