After being crowned King, Macbeth changes rather quickly. When he orders men to kill Banquo and his son, and the fact that he didn't even consult his wife about it, we know that he is losing his sanity. He is completely paranoid at this point. Then to make things worse, we see his murderous plans don't stop there. This is Macbeth speaking before sending murderers to kill off Macduff's family.
"From this moment
The very firstlings of my heart shall be
The firstlings of my hand. And even now,
To crown my thoughts with acts, be it thought and done:
The castle of Macduff I will surprise;
Seize upon Fife; give to the edge o' the sword
His wife, his babes, and all unfortunate souls
That trace him in his line."
He is so worried about the "Thane of Fife" that he's willing to destroy the whole family line. He murders innocent CHILDREN here. The readers definitely change their minds about his character. In the beginning at least he felt guilt about taking Duncan's life. He really struggled with that first kill. However, after he receives the crown, he is a new (and scary) man. He never turns back. He is going to keep the crown or die trying.