Macbeth is saying that there are a couple of problems with murdering King Duncan. First of all, in general, when you do something evil, it tends to come back and get you ("Bite you in the butt," so to speak.) Secondly, as a host AND as Duncan's subject, Macbeth should be protecting Duncan, not trying to kill him. Finally, as far as Duncan is specifically concerened, he has been a good king. He has not abused his powers and has been generous. Macbeth recognizes that the only reason he wants to kill Duncan is his own ambition to be king - Duncan does not deserve to die.
The first part of this soliloquy is discussed in the eNotes free Shakespeare Quotes section. Also, the modern English translation may be found at the link below:
Macbeth lists 7 reasons why killing Duncan would be wrong:
-vengeance - the killer will be killed (lines 8-12)
-kinship - you don't kill your relatives (lines 13-14)
-loyalty - you don't kill your king (lines 13-14)
-hospitality - a host doesn't kill your guest (lines 14-16)
-Duncan's goodness - you don't kill a virtuous king (lines 16-20)
-pity and horror - murder is unnatural to innocent humanity and to heaven (lines 21-25)
In the soliloquy he also mentions at the beginning how if there were no consequences resulting from the murder, he would risk it, not worrying about the future. (1-7) At the end of the soliloquy he mentions how ambition is his only motivation to kill Duncan. (25-28)
Notes taken from my Cambridge School Shakespeare copy of Macbeth.
Macbeth had been contemplating murdering Duncan but decides not to because:
- He risks punishment in afterlife
- Duncan had used his powers as King not harshly
- Macbeth is his host and should not bear the knife
- Duncan was a good man and had done nothing to deserve murder
- Macbeth is his relative
- Macbeth had realised that his only intentions for killing Duncan were his own ambition to become King
macbeth says the pros and cons about killing macbeth
Macbeth has been thinking about the murder and decides that he should not commit the crime for several reasons. First of all, Duncan never did anything to him. Second of all, Karma. The theme of this story is "Fair is Foul, And Foul is Fair." His soliloquey is backing this statement up.