The effect of Macbeth's entrance on stage after he has murdered Duncan is, in a word, dramatic--dramatic in that it accomplishes several purposes in moving the play forward. With his appearance, we learn that Duncan is indeed dead; Macbeth has followed through, as planned. Also, since Duncan's murder is not staged, Macbeth's recounting of events takes the audience inside Duncan's chamber and allows us to visualize and experience what just occurred there.
His entrance is dramatic, also, because it establishes Macbeth's immediate reactions after having killed the king. When we last saw Macbeth, he was witnessing bloody daggers in the air, expressing thoughts of great fear, and summoning up the courage to act:
Whiles I threat, he lives:
Words to the heat of deeds too cold breath gives.
When we next see him as he enters the scene, he is deeply shaken and full of anguish, not for the king but for himself. "Macbeth shall sleep no more," he cries. Thus his character is further developed. We know that his having killed Duncan is not a deed he will throw off easily; it will have emotional repercussions. Those repercussions are foreshadowed in Lady Macbeth's warning to him:
These deeds must not be thought
After these ways; so, it will make us mad.
Macbeth's appearance after killing Duncan advances and intensifies the drama because it gives us a glimpse of the madness to come.
When Act II, Scene II begins, Lady Macbeth informs the reader that Macbeth is carrying out the murder of King Duncan. But an interjection from Macbeth himself ("Who's there? What, ho!") suggests that he may not have carried out the deed, prompting Lady Macbeth to say that she wishes she had killed the king herself.
Macbeth's entrance, however, demonstrates to the reader that he has indeed killed the king. He enters carrying "bloody daggers," for example, and tells his wife that he has "done the deed." His entrance also hints at the mental anguish that both Macbeth and his wife will feel later in the play. Macbeth looks down at his bloody hands, claiming that they are a "sorry sight." He also tells his wife a story about how the servants woke up in the middle of the murder and began to pray. Macbeth's inability to say the word "Amen" made him worry because he needed God's blessing at that moment in time.
His entrance, then, is critical in not only confirming his status as a murderer but also setting the scene for his mental decline.