What are the similarities and differences between Hamlet's and Macbeth's moral decisions in the plays?
I am interested in considering what they should have done, instead of the action they chose to take. I just want to be clear that I understand the two plays and know the underlying meanings of each.
Similarities between the choices Hamlet and Macbeth make are for the most part superficial.
The catalysts for the plots in these two plays are supernatural, or at least possibly supernatural--the Ghost tells Hamlet to revenge his father's death and the witches suggest Macbeth will be king. And both plots involve the killing of a king--not a small thing. Assassinating a king is an act against God and, usually at least, an act against the natural order, since kings were thought to rule according to God's will (the divine right of kings). Macbeth, especially, since Duncan was a thoughtful, humble, and fair king, is in violation of everything right and good when he murders Duncan. This similarity is superficial, however, except for the fact that Hamlet must consider that others will think he has acted in a similar way as Macbeth acts, if he kills Claudius before all is revealed to Denmark.
In actuality, though, the similarities are superficial, because the Ghost and the witches, and Hamlet's motivations and Macbeth's motivations, couldn't be more different. Hamlet is a revenge tragedy, while Macbeth is not. The Ghost has the cleansing of Denmark in mind, while the witches want to wreak havoc in Scotland. The Ghost wants justice, but the witches want injustice and anarchy.
And Hamlet's motivations are just. He wants to avenge treachery. Macbeth is the treachery. Hamlet wants to kill the king that killed his father and is committing adultery with his mother. Macbeth wants to kill the king that stands in his own way of being king. Hamlet is motivated by love and justice (although his obsession with his mother's sex life does motivate him, as well), but Macbeth is motivated by ambition.
Hamlet's poor choice is to not kill Claudius in Act 3 once he knows the Ghost is telling the truth. He steps beyond his place in existence and tries to determine another human being's eternal salvation.
Macbeth's poor choice is to kill an innocent king for his own personal gain.
The difference between the two is further established by Macbeth's decisions for the rest of the play: he orders the deaths of Banquo, Fleance, and Macduff's family. Killing Macduff's family is particularly heinous, since it serves no political or military purpose.
I think it is very thoughtful to consider the actions of Hamlet and Macbeth as active choices that they make. This, of course, implies that there are other choices that they could have made. I'll give a few comparisons that spring to mind, to help in considering the question. But there is no one answer. You must discover what strikes you as important.
First, both Hamlet and Macbeth decide to commit murder. The circumstances and the amount of time spent debating this course of action are where they differ.
Macbeth is not alone in his choice. The determination and strength of his wife plays a huge part in his choice to kill the king. The morality of this decision is definitely in question. Macbeth reasons to himself that, since it was foretold that he would be king, how wrong could it be to help events along a bit? Again, Macbeth may have, acting on his own, been more cautious, but Lady Macbeth drives the action forward in order to strike while they have Duncan under their roof. In Macbeth, the murder happens in Act Two, scene one.
Hamlet's act of murder is also one of revenge: He must kill the murderer (his uncle) of his father. Like Macbeth, he is spurred by another to commit the act. In this case the instigator is the ghost of his father. Hamlet is very cautious in this regard. He questions whether the ghost is an agent of Heaven or Hell. He questions whether, in murdering Claudius, he might send him to Heaven, instead of Hell where he belongs. And he questions Action itself. Most notably, he alternates between chiding and laughing at himself for not being able to do that which he knows he should do. It takes him the entirety of the play to decide to act.
So here's a huge difference: Macbeth knows in his heart that the murder he commits is wrong, while Hamlet cannot decide whether it is right or wrong, and is really only able to commit the deed when he becomes enraged at the poisoning of himself and his mother at the end of the play.
It should be noted that morality is sometimes an issue of culture. Revenging the murder of family members is still common practice in many cultures today, and was a popular topic of plays in Shakespeare's day. The character in these Revenge Tragedies, whose job it was to avenge the death of a famly member, were considered the heroes of the plays and, once the act was committed, justice was considered to have been served.
So morally speaking, and as the question of what acts should have been committed by Hamlet and Macbeth, there are many things to consider, and, ultimately, the choice is yours as the audience member. What do you think they should have done?