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.
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.