Being king is great, according to Macbeth, and he wants to stay king. This is the problem. He cannot stay the king, and he cannot ensure that his chidren will be kings if Banquo and his son Fleance are alive. In Act III, sc i Macbeth acknowledges this when he says, "To be thus is nothing; /But to be safely thus."
Macbeth is slowly beginning to realize that the witches prophecies, which have proven true for him, will also prove true for Banquo and Fleance. He is king, but for how long? Macbeth notes,
Upon my head they placed a fruitless crown,
And put a barren sceptre in my gripe,
Thence to be wrench'd with an unlineal hand,
No son of mine succeeding. If 't be so,
For Banquo's issue have I filed my mind;
For them the gracious Duncan have I murder'd;
Now Macbeth is forced to continue the murdering and eliminate his friend Banquo and his son Fleance.