Macbeth's tragedy is that he understands the evil nature of his actions, but proceeds with them anyway in Macbeth.Discuss.
If you want to discuss Macbeth's knowing the evil nature of what he does in Shakespeare's Macbeth, you can look at a couple of passages that demonstrate his awareness.
After Macbeth is told he will be both Cawdor and king, and the prediction concerning Cawdor comes true, he reveals in an aside that he is already thinking of killing Duncan, thinking what it will take for him to become king, and these thoughts fill him with horror. In his aside in Act 1.3.133-145, he reveals that his thoughts "yield to that suggestion/Whose horrid image doth unfix my hair,/And make my seated heart knock at my ribs/Against the use of nature" (137-140). And he considers the thoughts "horrible imaginings."
Then in Act 1.7.1-28 Macbeth ponders the reasons why he should not assassinate Duncan. Macbeth worries about damnation in the afterlife if he goes through with the killing. He feels badly because Duncan has treated him well, and because Duncan has been a humble king. He also recognizes that his role as host to Duncan should result in his protecting his guest, not in his killing his guest.
All of the above reveal that Macbeth understands the evil nature of his actions. He recognizes that:
...I have no spur
To prick the sides of my intent, but only
Vaulting Ambition, which o'erleaps itself
And falls on th'other--
He closes his speech with the metaphor comparing his vaulting ambition to a rider jumping on to a horse and falling off the other side. He has nothing to spur him on--no good reasons (he's just contemplated all the reasons not to kill Duncan). He has only his vaulting ambition, which will result in his falling off on the other side of the horse, figuratively.
But, of course, Macbeth goes ahead with the assassination anyway. He knows the evil nature of what he's about to do, but he does it anyway.