For most of the play, Macbeth exhibits the behaviors of a tyrant and/or a fascist. His descent from loyal subject to murderous tyrant begins at the end of Act One, but he doesn't make the full transition until Act Two.
Prior to Macbeth's meeting with the witches, he had been a loyal subject to the king and a worthy and successful soldier. In fact, in Act One, Scene Two, the Sergeant informs Duncan of Macbeth's brave fighting in battle:
For brave Macbeth--well he deserves that name--
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valor's minion, carved out his passage, (I.ii.18-21)
Duncan responds, "Oh valiant cousin!" Duncan is so proud of Macbeth's brave exhibition that he grants him an addition title, the Thane of Cawdor. Shakespeare shows how a brave, loyal Thane (Baron) could succumb to greed and ambition at any costs. His heroism dies with his rise to power and eventual downfall. Macbeth was heroic in battle. This is noted in the first act. But with a little push from the witches and some manipulative encouragement from his wife, he abandons that heroism and loyalty.