How does Shakespeare present Macbeth's decision to kill King Duncan? In act 1, scene 7, what effect does it have when Macbeth says the word...

Macbeth in this scene tries to rationalize Duncan's murder as a quick, clean, one-time affair that will get him what he wants. Calling it an assassination, rather than a murder, helps Macbeth distance himself from the crime: an assassination is a political rather than a personal act. To call it an "assassination" is to say, in effect, "it's nothing personal, Duncan, just something I had to do to realize my own ambitions." 

However, while Macbeth...

Macbeth in this scene tries to rationalize Duncan's murder as a quick, clean, one-time affair that will get him what he wants. Calling it an assassination, rather than a murder, helps Macbeth distance himself from the crime: an assassination is a political rather than a personal act. To call it an "assassination" is to say, in effect, "it's nothing personal, Duncan, just something I had to do to realize my own ambitions." 

However, while Macbeth very badly wants the crown, he also feels a deep personal loyalty to Duncan. This loyalty makes the crime all the more difficult, no matter what he tries to tell himself about an "assassination." He wants to get it over with quickly and feels that if it is just one clean murder, it is worth it. However, worries crowd his mind. First, he thinks of the example such a murder or assassination would set for the people, giving "bloody instructions" to the masses and carrying the risk that the "poisoned chalice" of murder and mayhem would come back to haunt him. He also recalls that he owes Duncan loyalty and is bound by the code of the host to treat his guest well and with protection. As he puts it, he should "against his [Duncan's] murderer shut the door," not murder him himself. He also remembers what a "meek" [humble] and "clear" [honest] ruler Duncan has been and imagines what a shock it will be when the common people, who love him, find out he has been murdered. 

In fact, by the end of this long stream of thought, Macbeth has talked himself out the deed, and it takes Lady Macbeth's ruthless ambition to steel him to the task.