In Act I, the Bleeding Captain tells King Duncan of Macbeth's valiance on the battlefield. He and Banquo change the tide of the battle, and Macbeth deals with the traitor McDonwald with cruel mercilessness: he guts him. In short, he embodies the warrior's code; he is the emblem of comitatus (the King/Thane loyalty bond), and Duncan will soon bestow him with titles.
On the battlefield there was no downward spiral: he was always brave and cold-blooded. His fights with Young Siward and Macduff in Act V are no less valorous than his fight in Act I. He measures his worth with steel.
In terms of ambition, there was no downward spiral: he's always been this way. Before he murders Duncan, he speaks of vaulting ambition:
Secretly, he wanted to be Thane of Cawdor and King. It is only now that the latter is so close does he dare admit it to himself. The witches plant the seed, and his wife, the opportunist, begets the plan, but Macbeth has always coveted Duncan's title.
Macbeth's loyalty to King, kinsmen, and country breaks down the most. Just as soon as he guts a traitor in Act I, he becomes one. The old Thane of Cawdor was a traitor, and his title falls on Macbeth. Duncan is twice deceived, and by promoting Macbeth to Cawdor the King has sealed his own fate.