Why does Macbeth kill Duncan?

Traditional explanations are that:

a) Macbeth, with his vaulting ambition, had already secretly considered killing Duncan to take the throne (propounded by two of my teachers at school and the lecturer at uni) OR
b) The witches and Lady Macbeth urge him to do it (but is he really that weak?)

The problem is he knows it's evil and speaks against the deed. And he only discovers he's not going to be king after the great military victories in the first scenes.

It may be ambition and pique, but if he is such a great and loyal lord and, as he asserts how wrong it is, it still lacks credibility. To kill a man who has been so good to you, in a time when everyone is secure, to destroy everything with a personal and bloody regicide- in your own house? It doesn't make sense: he'd have to be unhinged, or missing the bit of the brain that deals with consequences.

It would work if Duncan was an idiot rather than a saintly and loved King OR if the decision to make the King's son was patently unjust and wrong for the kingdom - that is that Macbeth's claim was stronger. The problem is Malcolm is painted as a saint as well. The way Macbeth is written though he is far to self-aware.

So why has he embraced evil and a course he knows is wrong?

There are two possibilities I want to explore-
1 He is a great warrior but has other character flaws than vaulting ambition. What he does best is solve problems with violence, and for all his ability to reason and talk he is such a good killer that's his solution to an injustice. Also his flaws may include being so self absorbed he doesn't understand consequences or simply can't visualise them.

2. He wants the throne AND the witches and Lady Macbeth have either entrhalled him or created a space where somehow the murder seems to be acceptable all right - despite the angels of heaven calling against it. After all Lady Macbeth is the one who has called on the powers of evil to possess her.

Macbeth seems divided over the first killing and then is driven by his need for security for the others. I like the idea that somehow the witches and Lady Macbeth have conned or enchanted him to be blind to the outcomes.