Why did macbeth kill Banquo?

Lady Macbeth didn't make or influence Macbeth to kill Banquo, it was the worry of Macbeth that Banquo's blood line would rein one day, based upon the 3 prophecies by the witches. In fact Macbeth did not tell Lady Macbeth about the attack on Banquo and Fleance because Lady Macbeth and Banquo were close and Macbeth did not want to ruin the already fragile state of his wife.