How does macbeth's character change from valiant soldier to murder and tyrant?
Macbeth begins the play as a valiant soldier however as the play goes on he degenrates and becomes paranoid. How does he become a murderer and tyrant and at what point does this change really begin, and did macbeth already began planning to be king before the witches?
I know banquo says' why do you start?' after Witches first phropecy.
The above answer does a great job of covering much information toward an answer to your question. I'll just add bits and pieces to what's already been established.
When Banquo asks Macbeth
...why do you start, and seem to fear
Things that do sound so fair?...
he means why are you startled and why do you seem afraid of things that sound so good. The implication is that the idea of gaining more power startles Macbeth and makes him afraid. First, this suggests that Macbeth has thought of this before. Banquo does not take the witches seriously, he has not considered that maybe his heirs will rule Scotland one day. Macbeth takes the witches seriously and is frightened by their predictions. It could well be that Macbeth has considered this before and knows that his becoming king could mean only one thing. He knows the shortest way to the throne.
In short, no, we don't really have evidence that Macbeth was making any plans to be king before the witches predict that he will be king. At the same time, his reaction to the witches' predictions could be an indication that the thought had crossed his mind.
Macbeth, as you said, began as a superb soldier and wonderful warrior, but becomes a bloody tyrant and degenerates with the course of time. And the chief reason behind his degeneration is his ambition.
Once a person becomes a murderer and obsessive to achieve power, then to hold on it, he can do anything for the sake of power. His gluttony makes him paranoid and destroys all his good qualities gradually. it becomes difficult for him to come back from the path of hell he chooses for himself. Macbeth in Shakespeare's Macbeth, faces a very similar situation. He says: "To be thus is nothing, / But to be safely thus.", and in order to secure his kingship, he continues assassinating on and on. Later he utters, "I am in blood/ ... Returning were as tedious as go o'er." (act 3, scene 4). so, a brave warrior and an obedient royal employee becomes a 'butcher' very soon because of his inner devil. Here, we find a similarity between Marlowe's Dr. Faustus and Shakespeare's Macbeth.
Macbeth, as my reading of the play suggests, was already an ambitious man while his wife was greedy. What he needed was just an instigator, a spur. The witches' prophecy or equivocation worked like the spur. It is evident in the fact that, when the witches were uttering their prophetic statements, he had Banquo with him, and it was stated that Banquo's progeny would be successor to the throne. Still, it was only Macbeth, the Thane of Glamis, who took the "weird sisters"'s statements seriously. Banquo, unlike Macbeth, does not believe in the witches' prophecies blindly. Rather, Banquo is shown to say: "The instruments of darkness tell us truths/ Win us with honest trifls, to betray's/ in deepest consequence..." when asked by Macbeth: "Do you not hope your children shall be kings...?" (Act 1, scene 3). It is Macbeth's inner evil which, spurred by the witches, became more gluttonous. Later, his wife helps to the full growth of the ambitious tyrant inside him who could do anything for holding on the power.
So the reply to what you've asked: "did Macbeth already began planning to be king before the witches?" will be that, Macbeth is not directly shown to be plotting to be the king, but it is clear in Lady Macbeth's utterance to herself in act 1, scene 5, that both were aiming to reach the goal when she says: "Thou wouldst be great, /... / To have thee crowned withal".
At the beginning of the play, macbeth is a good, noble soldier, loyal to his king and ready to serve his king; But when the witches come and prohesize him, he starts to think that they are true and tries and hopes that it becomes true. it's not the witches who made the prophecies come true but macbeth who tried and did all the wrong things to make it true. It all begins when ross comes and says that he's become the thane of cawdor. macbeth then tells all about this to his ambitious wife. lady macbeth suggests him to kill king duncan! from then onwards macbeth starts to become a tyrant and a wicked person...