In short, he chooses evil over good. Macbeth somehow justifies murdering Banquo and Fleance, which demonstrates his deteriorating mental state. He did not struggle with murdering them as he did with Duncan.

He says:

For them the gracious Duncan have I murder'd;

Put rancours in the vessel of my peace

Only for them; and mine eternal jewel

Given to the common enemy of man,

To make them kings, the seed of Banquo kings! (III.i. 65-69).

Macbeth no longer stops to question his behavior or his motives. He simply moves forward with his plan. He plans the murders and make provisions for them on his own. The fact he does not need Lady Macbeth to prod him along illustrates his mental instability. His behavior even startles Lady Macbeth.

Macbeth...

Macbeth possesses a healthy amount of desire and ambition when we him early in the place. He allows himself to be influenced by the witches and his wife to the point that he loses his moral center. With all the talk about being king going on, Macbeth is wrapped up in the idea of it all and forgets what he is doing. After Lady Macbeth goads him into killing Duncan, Macbeth's countenance changes. He knows what he is capable of doing and once he realizes murder is an easy way to get what he wants, he cannot stop. The one thing he could not control was the slipping of his mind and this was what he needed to manage the most.
Work Cited

Shakespeare, William. Macbeth. New York: Washington Square Press New Folger Edition.