One could attempt to convince Macbeth that killing King Duncan will only lead to more bloodshed and anxiety, which can be psychologically damaging and stressful. With Macbeth as the illegitimate king of Scotland, his position and title will be in danger as Malcolm and other loyal followers of Duncan attempt to dethrone him. Macbeth will then be forced to defend his position as king by murdering his opponents. This increased anxiety and stress will be psychologically harmful as Macbeth will be forced to eliminate his enemies.
One could also make a religious appeal by informing Macbeth that committing such a heinous crime will have spiritual consequences. Killing a king is not only a crime, but is also a serious sin, which Macbeth will have to atone for in order to save his soul. Essentially, Macbeth risks corrupting his soul and suffering for an eternity in Hell if he kills Duncan.
Given The Witches' prophecies, one could also argue that Macbeth will not have a legacy even if he does attain the title of king. The witches predicted that Banquo's descendants would become kings, which reveals that Macbeth will not have a lasting legacy.