As Macbeth considers the idea of killing Duncan, he articulates the reasons most important reasons for not killing Duncan:
1) Duncan is his kinsman--Even though it was certainly not unusual for the aristocracy in Scotland to kill each other to gain positions, the killing of a family member, no matter how distant, was considered among the most serious sins and breaches of duty.
2) Duncan is Macbeth's King--The killing of a king is the highest level breach of duty and loyalty, particularly if one is killing the king simply to sastify one's ambitions. More important, Duncan is described generally as a decent king (slightly weak, but still a good king), so Macbeth is not even doing the realm any good by killing Duncan.
3) According to the code of honor that prevailed during this period, one has a duty to protect a guest in one's home at all costs. To kill a guest, especially a sleeping guest, is the most serious type of betrayal someone can commit.
If Macbeth had been left to his own devices, even with his incredibly strong ambition to be king, his analysis of the reasons not to kill Duncan might have kept Duncan alive. The appearance, however, of Lady Macbeth, whose ambition is at least as strong as Macbeth's, changes the dynamic entirely.
Lady Macbeth sees Macbeth's resolve weakening--based on the reasons above--and she accuses him of cowardice, questions his manhood, and even tells him that if she had promised to carry out such an act, nothing could stop her from accomplishing it. Critics have long debated whether Macbeth or Lady Macbeth was more responsible for Duncan's killing, but there is no doubt that Lady Macbeth's forceful resolve is just enough to tip the scales of doubt in Macbeth's mind, resulting in Duncan's murder.
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