At the time Duncan appoints Malcolm as the Prince, Macbeth has already had his first encounter with the three witches. The seed has been planted in his mind that he will become king. When this happens, the only character Macbeth is concerned with killing is Duncan. Now, because Duncan has appointed Malcolm, meaning he is next in line for the throne, Macbeth has to deal with Malcolm as well. Because Malcolm is appointed prince, Macbeth's plan now has to include Malcolm, which Macbeth successfully concludes that because of this appointment, Malcolm is now eager to become the king. Malcolm could become suspect for the murder of Duncan. If done correctly, Macbeth's plan can take care of Duncan and Malcolm.