Macdonwald was a traitor to King Duncan and Scotland.
Macdonwald was the head of the opposing army Macbeth faced in the events that took place right before the play.
The merciless Macdonwald—
Worthy to be a rebel, for to that
The multiplying villainies of nature
Do swarm upon him—from the western isles
Of kerns and gallowglasses is supplied… (Act 1, Scene 1)
Thane of Cawdor, Macdonwald is supposed to be an honorable gentleman. Instead, he turns traitor and faces off against Macbeth. Macbeth is able to defeat him, and it is this brave and honorable feat that earns Macbeth the title of Thane of Cawdor, replacing the man he defeated.
Macdonwald does not give up easily. He is described as "merciless." He seems to continue fighting even against Macbeth, his former countryman. This is why Macbeth “unseam'd him from the nave to the chaps,” or basically sliced him down the middle.
The irony of Macbeth gaining his first title by way of killing a traitor is that Macbeth himself becomes a traitor. With this act, he proved his loyalty to Duncan, but at the same time he is soon scheming to replace Duncan because of the witches' prophecy that he will be king. Macbeth's killing of Macdonwald is significant because it makes the first prophecy come true, thus setting in motion the rest of the events of the play.
Macdonwald was the leader of the rebels fighting against King Duncan's armies, and indeed was slit down the middle and beheaded by Macbeth. He was not also the Thane of Cawdor. Once Macbeth defeated Macdonwald, the Thane of Cawdor, in league with the king of Norway, helped make it possible for Norway to invade Scotland, hoping to take advantage of Macbeth's weary army, to defeat them. Cawdor was the traitor and the Thane Duncan had trusted, and it is, of course, brilliantly ironic that the title is next bestowed upon Macbeth, whom Duncan also trusts.
Macdonwald is described by the injured Captain in Act I, scene 2, as being "merciless" in battle and possessing all the villainies of nature that one would expect to find in a person capable of the evil of rebellion against the throne. He must have been Nasty! He is slit up the middle and beheaded by Macbeth. Once the rebels are thus defeated, the King of Norway attacks, aided by the Scottish traitor, the Thane of Cawdor. The Thane of Cawdor and Macdonwald are two seperate characters. Macbeth fought two separate armies just before the play begins. Actually, he's still fighting the Norwegians at the play's opening.