I would argue that Macbeth is already considering killing Duncan before he goes to see Lady Macbeth.  He sent his letter telling her what the witches told him as a kind of confirmation. 

Consider Macbeth’s reaction to finding out Duncan did not name him his successor.  He gets very frustrated, and says that it is a “is a step/ On which I must fall down, or else o'erleap” because it is in his way.

Stars, hide your fires;

Let not light see my black and deep desires:

The eye wink at the hand; yet let that be

Which the eye fears, when it is done, to see.(60) (Act 1, Scene 5, p. 18)

This aside takes place in Act 1, before Macbeth goes home to talk to his wife.  Lady Macbeth definitely spurs him on though, because he later wonders if killing Duncan is the right choice and she talks him into it.

See here for the quote

Summary

Character Analysis