The witches' prophecies are ambiguous, and they can be interpreted in different ways. Macbeth chooses to interpret them as a sign that he will become king. However, he could have also interpreted them as a warning that he should not become king. He could have chosen to ignore the prophecies altogether.
Macbeth is a complex character, and his decision to murder Duncan is not simply the result of the witches' prophecies. He is also motivated by his own ambition, his desire for power, and his fear of losing what he has. The witches' prophecies provide him with an opportunity to fulfill his ambitions, but they do not force him to act. He chooses to act on his own free will.