Describe Macbeth's character development.

We can chart Macbeth's descent into absolute moral corruption by the way the murders for which he is responsible are shown (or not shown, as the case may be) to the audience. For example, Macbeth kills during battle, but these killings all take place far offstage and in the context of a battle (which seems to excuse them to a certain extent). We hear of them only, never seeing them or any evidence of them. The next killing, of King Duncan, Macbeth's friend, guest, and kinsman, takes place just offstage, and, this time, the audience actually sees the blood on Macbeth's hands and hears about it from his own lips. It is a brutal murder, death by stabbing, and the audience feels much nearer to it, increasing its horror. The next murder and attempted murder of Banquo and Fleance, respectively, take place onstage, for the first time, and the audience actually watches as Banquo is cruelly killed and as the murderers try to kill his young son as well. That we actually see this murder and that it includes an attempt on a child's life, makes this one even more reprehensible than the last. Finally, when Macbeth arranges for the murders of Lady Macduff and her several children, these too take place onstage, in full view of the audience. Not only does this murder happen right before us, but it also serves no purpose whatsoever. In killing during battle, Macbeth fought to defend king and country; in killing the king, he usurps that title and status; in killing Banquo, he attempts to prevent Banquo's descendants from taking the crown; however, in murdering a woman and her children, Macbeth gains nothing. The extent of the violence and its proximity to the audience grow each time Macbeth kills, providing evidence of his character's development for the much, much worse.

From the outset of the play Macbeth is ambitious; his zeal in defending king and country is praised by the injured captain, who describes the courageous and bloody killing of the traitor, Macdonwald. He is happy to receive an additional title from Duncan. The first encounter with the Weird Sisters turns his ambition in a new direction as he quickly comes to the conclusion he must remove any obstacle that would prevent him from attaining the throne of Scotland.
Even so, his wife's plan to kill Duncan unsettles him, and he briefly considers backing out of their planned regicide. This scene, however, marks the end of Macbeth's loyalty to Scotland and his king and the end of his morality.  Once Duncan and his chamberlains are dead, Macbeth's ambition continues to grow unchecked. Acting independently of Lady Macbeth, Macbeth seems to have no reservations about arranging the deaths of Banquo, Lady Macduff, and Macduff's family. He attempts to have Fleance killed as well because he fears that Fleance could one day interrupt his reign. It could be said that once he has killed Duncan, Macbeth's ambition is forever wed to ruthlessness and an intensified drive to remain king.  
The death of Lady Macbeth does little to alter Macbeth's trajectory. He kills Young Siward and plans to kill Macduff, as well, in his attempt to remain in power.  Macbeth's ambitiousness follows him to his final moments; he tells Macduff he will neither yield nor fall on his sword. Among his finals words, he proclaims, "I will try the last."