why Macbeth kills Duncan
There are several reasons for his decision. Macbeth has been told by the witches that he will eventually become king. If Duncan is already king, how can Macbeth become king? Answer: kill Duncan. However, this is Macbeth's first mistake. The witches operate on the basis of fate--it is destined for Macbeth to become king. However, Macbeth takes matters into his own hands (rather than just letting events play out as they will), and that disrupts fate's system. (Or you can argue that it was fated that Macbeth would kill Duncan and that's what the witches were predicting when they said he's become king.)
Lady Macbeth also encourages Macbeth to kill Duncan. She is almost most status and power hungry than he is. She calls his manhood into questions, saying if he was a true man, he would do this. She uses all sorts of feminine manipulation to persuade him into killing Duncan--and Macbeth doesn't want to see like less of a man to his wife.
When the deed is actually to be committed, Lady Macbeth said she would have killed Duncan, but he looked too much like her father when he was asleep. Therefore, it leaves Macbeth to actually do the killing.
Some reasons MacBeth would want to commit regicide and kill King Duncan:
1. To gain power over all of Scotland and advance his station in life.
2. To achieve the crown.
3. To fulfill the prophecy.
4. To look like a "man" in his wife's eyes.
Some reasons NOT to kill Duncan include
1. Murder is against natural law.
2. Killing a King will disrupt the Elizabethan cosmology and chaos would soon result.
3. Disorder, chaos, and unnatural acts will be loosed on the world.
4. The Weird sisters may not be a reliable source of information or prophecy. Why should he trust them?
5. Guilt and a stained conscience will inevitably result in psychological ailments.