The simple answer is that Macbeth is initially a hero, but he becomes a dangerous, murderous man.
Before we even meet Macbeth, the Sergeant informs Duncan of Macbeth's brave deeds:
For brave Macbeth - well he deserves that name,
Disdaining fortune with this brandish'd steel,
Which smoked with bloody execution (I.ii.18-20)
Duncan is so grateful to Macbeth for winning the battle that he awards him the title of Thane of Cawdor after the previous Thane committs treason. So, from all that we know prior to Macbeth's meeting with the witches, he is a brave soldier and is loyal to Duncan. In short, he is heroic in the beginning.
However, it doesn't take long for him to devolve into a very dangerous man. The seed is planted in his mind that he will be king. He communicates this in writing to his wife. She is convinced that Macbeth will need encouragement to become ambitious enough to follow this path. And indeed, she does this in Act I, Scene 7. By Act II, Scene 2, he has killed Duncan and is so plagued by fear and anxiety that he becomes even more dangerous. He will do anything to protect himself. It is important that he is initially presented as a heroic, loyal subject to the king. His relatively quick transition to a dangerous man shows how greed and power can corrupt very quickly.