In Act I of Macbeth, Macbeth is presented in the following ways:
Brave, courageous, but bloodily cruel: As you say, he and Banquo turn the tide for the Scottish against Norway and Ireland. Even before Macbeth meets the witches he guts Macdonlwald from navel to neck, decapitates him, and place his head on a pike as a show of force.
For brave Macbeth--well he deserves that name--
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valour's minion carved out his passage
Till he faced the slave;
Which ne'er shook hands, nor bade farewell to him,
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements.
In short, Macbeth's response to every problem is bloody violence. While Duncan rewards him for this in Act I, Macbeth's thirst for blood only continues until "blood will have blood."
Powerful: Macbeth is already a titled general in the Scottish army; he's Thane of Glamis. Because of his valor in battle, King Duncan requests a stay at his castle to bestow him with Thane of Cawdor.
Ambitious: The witches plant the seed of "King" in his mind, not that he did not secretly wish this anyway. After he tells this to Lady Macbeth, she plans how it may become a reality. All the while, Macbeth uses these women as advisers to rationalize his "vaulting ambition." Macbeth's politics are Machiavellian, and he uses the "ends to justify the means" in attaining the crown. In other words, he will kill his own king, kinsmen, best friend, and even children in pursuit and defense of the crown.