Point of Decision

After killing Duncan, Lady Macbeth sees Macbeth with blood on his hands.

One thing my group talked a lot about in our discussion today was the reasons behind Macbeth's decision to murder King Duncan. It was clearly a choice. We just had a hard time deciphering his motivations.
Anyways, I was reading through the first scene again, and I found a soliloquy that might contain some reasoning behind Macbeth's actions.

This supernatural soliciting
Cannot be ill, cannot be good. If ill,
Why hath it given me earnest of success
Commencing in a truth? I am Thane of Cawdor.
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
Against the use of nature? Present fears
Are less than horrible imaginings.
My thought, whose murder yet is but fantastical,
Shakes so my single state of man
That function is smothered in surmise,
And nothing is but what is not.

In this soliloquy, we can see Macbeth start to justify the murder he has committed already in his imagination. He says it "cannot be ill, cannot be good." So what is it then? He knows well enough that the witches have been right thus far. It's almost as if he wishes to prove all their words. 
Perhaps Macbeth has a fatalistic view, he does the deed because it has been foretold and there is no ignoring one's fate. This reminds me of Oedipus Rex a little. Oedipus brings about his fate despite doing everything to avold it. Macbeth does not possess the desire to avoid his fate.
Also, the witches do not tell him how his reign will end, only that Banquo will father a line of kings.
The introduction in the book also talks about Lady Macbeth being an influence on him. When she hears that Macbeth could become king, she doesn't hesitate to sell out.
My group also wondered if Macbeth was predisposed to the murder in some sense. He was at war for a long time. It's very possible that a few more dead bodies would not even cause him to blink.
What's interesting about the soliloquy is that Macbeth does acknowledge to a certain extent that what he plans is wrong. He talks about the horrid image of murder "unfixing" his hair.

So what is Shakespeare saying? It could be any number of things:

• You cannot escape your fate, so why try?
• Sometimes men do bad things because of our baser nature.
• Some men will do anything to achieve their ends.
• We as humans have a compulsive tendency towards curiosity.

He might be saying all or none of these things, but the point is that Macbeth causes us to consider morality and the line between ambition and evil.