What does Macbeth mean when he says, "We are yet but young in deed" in Act III, scene 4?
Part of the genius of Shakespeare’s play Macbeth is the depiction of the main character’s moral descent into evil and debauchery. After his wife, Lady Macbeth, convinces him to commit the first murder (King Duncan), Macbeth begins to morph into a different person—one who is obsessed with maintaining his position and eliminating his enemies.
When the line “We are yet but young in deed” occurs, Macbeth has just concluded a very difficult night in which he repeatedly saw the ghost of Banquo (whose murder he had recently arranged) appear during a banquet that included many other Scottish noblemen. The murder of Banquo is Macbeth’s second, and it is significant because it shows that Macbeth is willing to sacrifice a former friend and “right-hand man” to safeguard his newly won position as king. When he says that “We are yet but young in deed,” he is not referring to just any “deed,” but to bloodthirsty acts of cold-blooded murder. The following lines illuminate Macbeth’s attitude toward his own recent “deeds”:
I am in blood
Stepped in so far that, should I wade no more,
Returning were as tedious as go o'er.
These lines show that Macbeth is fully aware of the immorality in which he is now immersed. He holds no illusions about what he is doing, and he knows that he has gone too far down this evil path to turn back now. Since he is “young in deed” he realizes that he will have to commit more atrocities in the near future. In fact, his most atrocious act occurs in the next act when he commissions the murders of Macduff’s wife and children.
This line, as well as the few that precede it, show just how comfortable Macbeth has become with committing acts of grave violence. Before the murder of Duncan, it was Lady Macbeth who seemed to proceed without conscience or guilt; she scolded Macbeth again and again, after he murdered Duncan, for being cowardly and thoughtless. Further, he had said that he'd never be able to sleep again since he murdered Duncan while the king slept. Now, however, Macbeth seems to feel relatively little after the murder of his former best friend, Banquo. He sounds quite lackadaisical when he says, "Come, we'll to sleep." He then implies that his odd behavior was only the result of his inexperience with committing acts of violence, although his final line, "We are yet but young in deed," does make it sound as though Macbeth's level of comfort with murder and bloodshed has increased significantly.
It's as though Macbeth's tolerance for violence goes up, now that he's killed Banquo. After he murdered Duncan, he couldn't sleep, but now that he's had Banquo killed, he's ready to go to bed without further ado. After killing Duncan, it seemed as though Macbeth would never want to lift a finger to hurt another person again—so distraught and regretful he seemed to be—but now he acknowledges and accepts that they will likely have to commit more and more acts of violence to hold on to the power and position they have gained.
This statement comes at the end of the scene in which Macbeth has seen the ghost of the recently-murdered Banquo. Deeply disturbed, he is reassured by his wife that the specter was only a figment of his imagination. Eventually, Macbeth agrees, and he says that because he and his wife have not much experience in murder and intrigue, it is perhaps natural to be haunted by such visions:
Come, we'll to sleep. My strange and self-abuse
Is the initiate fear that wants hard use.
We are yet but young in deed.
By "initiate fear" he means that the two are inexperienced. Read in context, this quote could suggest that Macbeth recognizes that more murders will be necessary to maintain his position. They are "young in deed," but Macbeth doesn't seem to think they will stay that way.