Macbeth is indeed portrayed as a brave and noble warrior at the beginning of the play, when the sergeant describes, in gory detail, his actions in the battle against the forces of the rebel Macdonwald:
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution...
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements.
King Duncan describes Macbeth as a "worthy gentleman" and a "valiant cousin," and it is clear that the title character is willing to put his life on the line. This actually makes his wife's challenge to his courage, when he vacillates over the murder of Duncan, all the more poignant: We know Macbeth is not a coward, so it is a sense of right and wrong that causes him to hesitate before murdering the monarch.
So it is Macbeth's moral decline that is the most striking aspect of the plot. By the time he orders the murders of the Macduff family, he is a bloody tyrant. He is still not afraid to risk his life, and declares his willingness to fight Macduff to the death even after he finds out that the witches' prophecy is coming true. But he has lost (albeit not without some agonizing and residual guilt) any notion that right and wrong should guide his actions.