That he has become more comfortable with killing in order to keep his throne.
At first, Macbeth was worried to murder King Duncan to gain the throne; now, he is perfectly willing to murder his own friend, Banquo. He murders Banquo, and tries to murder his son, too, because the prophecy stated that Banquo's son and all his sons would be the future kings of Scotland; Macbeth wanted his descendants to reign.
Hope this helps!