In Macbeth, how and why does Macbeth turn from a warrior into an evil murderer?
Well, some people might argue that your question in itself is a bit of a paradox: surely all warriors that kill people are murderers! Whether they're evil or not is a different question, but your question draws out one of the key ironies of the play.
When we first hear about Macbeth, we hear that he's been "unseaming" people (literally, cutting them open at the seams) on the battlefield, and bathing himself in "reeking wounds". He's a terrifying soldier.
Yet when he comes in victory, and happens upon the witches, they make a prophecy that he will become king. He's immediately drawn:
Stars, hide your fires. Let not light see
My black and deep desires...
Macbeth has already thought about murdering the king, it seems, but the witches push the thought to the front of his mind. And, when he writes to his wife, she resolves to remove "all that impedes thee from the golden round". She persuades him to murder Duncan, and chastises him when, at one point, he claims he's going to go no further with the murder.
So it's the women - the witches and Lady M - that make sure he kills Duncan. But, it seems, his "black and deep desires" were there anyway. And that he reacts so badly - losing sleep, becoming paranoid, seeing visions - to one murder is the real irony. Macbeth is a ferocious, bloodthirsty warrior on the battlefield, but he can't kill one old man in his own house. It's all in how you think about it...
The difference between a proud warrior and a murderer can be summed up in one word-- honor.
So, ask yourself, when did MacBeth lose his honor? That will be the moment you can specifically say that he changed.