Read Macbeth's opening speech. List all the reasons he gives for not killing Duncan and the single reason he gives in favour of killing Duncan.
I assume you're speaking of Macbeth's first speech in Act I scene vii. Here is a soliloquy in which Macbeth is reconsidering their plan to murder Duncan. He lists several reasons why he should not kill Duncan. They include the following:
Macbeth says he's not only related to him (reason number one), but he's also his host (reason number two) and should be planning how he can best protect him, not how he could kill him. Reason number three is that Duncan is a good man who does not deserve such a fate. He praises Duncan's meekness and his virtues, noting that all of nature will mourn him--and damn or condemn the murderer--with a flood of tears when he's gone.
Finally, Macbeth finished lamely with his only motivation for killing Duncan:
Simply put, there are many reasons NOT to kill Duncan and only reason TO do so--his own selfish ambition. Shakespeare's Macbeth is the story of this very concern--Macbeth's arrogance and ambition in the face of reason and right.