We are never told explicitly, but in reality it would have been due to his commitment to Scotland in battle
The witches want Macbeth to become king because they enjoy causing chaos and influencing human lives. By propelling Macbeth towards kingship, they foresee the disruption and downfall that will follow, satisfying their own sinister desires.
Scotland
In response to Macbeth's questions, the witches prophecize that Macbeth will become king and encourage him to take action to make the prophecy come true. They manipulate his ambition and plant seeds of greed and paranoia, which ultimately lead to Macbeth's downfall.
The three witches told MacBeth he was going to become king. He decided this needed a little proactivity on his part. Prompted by his wife, he killed Duncan to move things along a bit faster
To make sure that Banquo's son wont become king as the witches prediccted.
Macbeth wanted to kill Banquo and his son because he feared that they were a threat to his throne. The witches had predicted that Banquo's descendants would inherit the throne, which posed a risk to Macbeth's own lineage. By eliminating Banquo and his son, Macbeth believed he could secure his power.
it means that one should use what ever they have to get where they want to be, to Macbeth this means that he should kill his king in order to become king
Hecate wants the witches to create a magical potion to deceive Macbeth and lead him to his downfall. She wants them to use their powers to manipulate Macbeth into feeling overconfident and secure, which will ultimately result in his defeat.
She was excited, obviously, and worried that Macbeth would not want to become king by the most obvious method--by murdering Duncan.
Assuming you meant when Macbeth sends the two assassins to murder Banquo and Fleance, it is because he doesn't want Banquo's children to become kings like the witches predicted. It only worked half way, because yes they did kill Banquo, but not Fleance.
Macbeth wrote a letter to his wife explaining to her all of what the witches had said. If he did not want her to be his "partner in greatness" he would have kept that knowledge to himself.
Banquo wants to discuss the witches' prophecies with Macbeth and their potential significance. He also wants to share his suspicions about Macbeth's recent actions to see if he can uncover any truth behind Macbeth's sudden rise to power.
king duncan was the king of demark which in the past was in a war with england, so in the play they didnt want to make demark look like bad people so they replaced the demark people with danes