What symbolizes the end of the righteous, brave, and loyal Macbeth and the birth of his evil and murderous counterpart?

At the beginning of the play, Macbeth is presented as a courageous general who fears nothing and is so loyal to his king and country that he would face any enemy to protect them. He is deemed honorable and seen as worthy of the king's admiration and reward.

It is Macbeth's overriding ambition that makes him evil. He is obsessed with the idea of being king of Scotland. Macbeth realizes he will not become liege by normal succession because King Duncan has two male heirs, Malcolm and Donalbain, who precede him in their claims to the throne. Furthermore, Duncan had already named Malcolm next in succession by bestowing him the title 'Prince of Cumberland.' Therefore, the only manner in which Macbeth can achieve "the golden round" is to assassinate the king and his sons and then claim the title.

What most profoundly symbolizes the beginning of Macbeth's descent into evil is his response to Ross's announcement that King Duncan bestowed the title 'Thane of Cawdor' to Macbeth. Prior to this, Macbeth and Banquo had an encounter with the witches who greeted him as follows in Act 1, scene 3:

First Witch
All hail, Macbeth! hail to thee, thane of Glamis!

Second Witch
All hail, Macbeth, hail to thee, thane of Cawdor!

Third Witch
All hail, Macbeth, thou shalt be king hereafter!

After confirming that he had indeed been awarded the prestigious title, Macbeth says the following in an aside:

Glamis, and thane of Cawdor!
The greatest is behind.

His response confirms his belief that the greatest hurdle in his desire to be king has now been overcome. The road ahead would now be much easier. His joy at this makes him believe the witches' primary prediction that he "shall be king hereafter" must naturally follow. It was his destiny. It is for this reason that Macbeth informs Lady Macbeth of the joyous news. He then starts deliberately considering Duncan's murder once he is back at his at his castle, as shown in Act 1, scene 4:

If it were done when 'tis done, then 'twere well
It were done quickly: if the assassination
Could trammel up the consequence, and catch
With his surcease success; that but this blow
Might be the be-all and the end-all here,
But here, upon this bank and shoal of time,
We'd jump the life to come. 

Although some may argue Macbeth still possessed some honor and good qualities at this point since he later told his wife that "we will proceed no further in this business," Macbeth's malice bred from ambition has already been unlocked. It was much too easy for Lady Macbeth to persuade Macbeth to continue with their plan. Macbeth's doubt about the plan was borne more from fear and uncertainty than from virtue.