Macbeth's positive and heroic qualities are stressed at the beginning of the play: he is a great warrior and spoken well of by everyone. Up to that point it seems his conduct has been above reproach. However, he has a tragic flaw - he has ambition to become king, and he is unable to resist this ambition, even if it leads him to committing murder. Perhaps the most tragic aspect about his character is that he is all too aware of his own weaknesses and the evil into which he falls, but he allows himself to be persuaded by his even more ambitious wife. However this more or less unhinges him. He hallucinates before and after Duncan's murder and is unable to resist the slide into further depravity, killing, or trying to kill anyone whom he feels might thwart him and his ambitions.But he ends up acting out of sheer desperation at his own fall from grace, he feels that nothing can save him anymore. However at the very end of the play he regains some of his honourable demeanour as a fearless warrior, fighting to the end although he knows all is lost.
Macbeth is essentially a man of high standing, who however becomes prey to negative external forces (the witches and his wife's aggression) and allows his own weaknesses to overcome him, leading to the ruin of his once-noble nature. His awareness of his own fall is what lends the play its tragic intensity.