In Macbeth, how does Macbeth show commitment?

Macbeth is committed when he shows that he is willing to do just about anything to hang on to the power and status he gained when he killed Duncan.  The Weird Sisters told Banquo that he would never be king but that he would father kings; his descendants, then, would rule.  This angers Macbeth because he doesn't like to think that he went to all this trouble to make royalty out of someone else's children.  He says, 

To be thus is nothing,

But to be safely thus. Our fears in Banquo

Stick deep, and in his royalty of nature

Reigns that which would be feared. (3.1.50-54)

Macbeth feels that it isn't worth being king unless he feels secure in his position; he cannot relax.  Therefore, he determines to kill both Banquo, so he can never have more children, and his living son, Fleance, so that he can never reign or produce heirs of his own.  It's hardly admirable commitment, but Macbeth is committed to holding on to power nonetheless.  Further, he is willing to do more yet in order to retain his power, telling his wife, "We are yet but young in deed" (3.4.150).  In other words, he feels that they have a great deal left to do to secure their throne from threats.