Lady MacBeth is prone to all the weaknesses of human conscience. She talks big to MacBeth because she is trying to encourage him and is excited by how palpably near power is. But when the great test comes, the murder of King Duncan, she cowers. Earlier she had said, "The raven himself is hoarse,/ that croaks the fatal entrance of Duncan/ Under my battlements." Shakespeare often uses the raven to denote the death of Royalty (such as in Hamlet when Hamlet says, "Begin, murderer;/ Leave thy damnable faces and begin. Come, the/ Croaking raven doth bellow for revenge.") Lady MacBeth acts as though Castle MacBeth were hers, and so she the ruler of it. She sows the seed of discord in MacBeth's ear, and together they plot the murder. When her part is up, she says in an aside, "Had he not resembled/ My father as he slept, I had done 't." This sounds suspiciously like, 'Well, I wanted to. But you see..." She's fishing for excuses as to why she did not do it, which smacks of a conscience. I think that she later feels a great deal of guilt for having thrown down the throne of Scotland.
Shakespeare has done this in other plays. Richard III involves two murderers, both of which talk big to their employer, but when the time comes to do the job, they have regrets, and one of them even refuses to continue.
As to the occult, I think Shakespeare was just trying to get his audiences attention. And you have to admit, it is interesting. Many religions despise witchery and in Shakespeare's time, "witches" were still being burned at the stake. But MacBeth was also one of the first plays Shakespeare wrote after the coronation of King James I of England, the VII (?) of Scotland. James could trace his ancestry back to Banquo. Shakespeare plays with history a little but showing how the enemy of Banquo dabbled in black magic adds legitimacy to the rule of a Christian King whose only great accomplishment was a translation of the bible into English. (the King James Bible)
English writing has done this before. Think of the Arthurian tales and Robin Hood.