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.