I don't know if you would call it greed -- I would be much more likely to call it ambition. Greed implies a desire for money -- ambition is more about getting power. I can see ambition in Macbeth's murder of Duncan but not greed (at least not greed for money).
The reason Macbeth kills Duncan is because Macbeth wants to become king. This, to me, is a grab for power, not for money.
So, if you define greed as a desire for power, then it takes place because Macbeth kills Duncan so that he can become king. If you define greed as a desire for money, I don't think it is there.
I agree, greed doesn't play into Macbeth's motivations. Greed for power, perhaps, but Macbeth doesn't even seem to have all that much desire in that direction.
Lady Macbeth, though, now that's a whole different cauldron of baboon's blood. Lady Macbeth is the one with the greed for power, the need to be Queen, to be more than what she is. She's the one who, when Macbeth returns victorious from war, she's the one who chastises him with her tongue prodding him to grab the thrown by murder. Remember he says to her (Act 1, Scene 4):
We will proceed no further in this business:
He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon.
Well, OK, but your wife, Mr. Macbeth, has other plans...
I agree with the previous posters if we take the definition of greed literally:
A selfish or excessive desire for more than is needed or deserved, especially of money, wealth, food, or other possessions.
The important thing to note here is the second half of the definition, an addendum that need not be applied: "especially of money, wealth, food, or other possessions." Yes, Macbeth has no desire (in the text at least) for any of those things. However, those are not necessary to constitute greed. In fact, I laughed when I read the two examples given for greed:
His greed was his undoing.
What drove them was their ambition, their greed for power.
I found it very ironic that a basic definition of ambition as "greed for power" was found right under the definition of greed! You can, then, have greed for something other than money, wealth, food, or possessions.
In sum, if ambition can be renamed as "greed for power," then greed can certainly be argued into the killing of Duncan, . . . for Macbeth himself agrees he has "vaulting ambition."