Macbeth's character is introduced in Act I through the description of his most recent heroic deeds in Scotland's war with Norway. A captain who comes from the field tells King Duncan of Macbeth's raw courage and determination in battle: Macbeth's sword "smoked with bloody execution" as he fought against overwhelming odds to defeat Duncan's enemies. Macbeth is presented as a loyal soldier and supporter of the King, a man of unprecedented valor, and for these attributes, Duncan loves and respects Macbeth and rewards him.
The witches' prophecies, however, incite a change in Macbeth's sterling character. His ambition is aroused as he contemplates himself as King of Scotland. When Duncan names his son Malcolm as his heir, Macbeth's immediate reaction is that Malcolm now stands in his way to the throne. Macbeth's course is set; although he turns away from it momentarily and ruminates on the horror of the crime he will commit, he proceeds in the plan to murder the monarch. Ironically, for one so strong and brave in battle, Macbeth shows fear of what is to be done and its possible consequences; his resolve seems weak in contrast to Lady Macbeth's.
In the conclusion of the first act, by the time Duncan arrives at Macbeth's castle, Macbeth's thoughts reveal him as the deadly conspirator he has become:
I am settled, and bend up
Each corporal agent to this terrible feat.
Away, and mock the time with fairest show:
False face must hide what the false heart doth know.
The valiant, loyal soldier who was willing to sacrifice his life for his king has become a traitor, conspiring with his wife to murder Duncan and steal the throne of Scotland.