What does Macbeth claim is his only motivation for the assassination?
Macbeth does outline numerous reasons that might compel him not to kill Duncan: he knows the murder will not instantly make him king -- there will still be more to do; he worries that his violence might come back to bite him in the end; then, Duncan is his kinsman, his king, his guest; finally, Duncan is a really good guy and fair ruler. He says, "I have no spur / To prick the sides of my intent, but only / Vaulting ambition, which o'erleaps itself / And falls on th' other--" (1.7.25-28). He means that his only motive to commit the murder is his great ambition which propels him, perhaps, further than it should. This, then -- his ambition -- is the one motivation for Duncan's assassination. However, his pride does seem to come into play as well. After Macbeth has discussed his ambition, he actually tells his wife, "We will proceed no further in this business" (1.7.34). He tries to back out of their plan, suggesting that ambition is not enough of a motivation. It is not until she wounds his ego, calling him a "coward" and claiming that he is no longer "a man" if he is unwilling to go through with it, that he relents and once again agrees to the plot.
Macbeth says in his speech at the beginning of Act I, scene 7, that there are many reasons NOT to kill Duncan - Duncan is his cousin, and a good and well-loved king, and his guest. However, Macbeth admits that the reason he still wants to go through with it is his "vaulting ambition" that overleaps his concerns.