why is being king not good enough for macbeth? in the book macbeth sene 3 act 1

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.