Macbeth is a play about death, deceit, and corruption. At the center of all this is Macbeth and his wife, Lady Macbeth. As the play progresses, their relationship changes dramatically as a result of how each of them handles their emotions following King Duncan's murder.
In the beginning of the play, Lady Macbeth is a strong, domineering person. She seems able to coerce Macbeth into doing things that he would not do on his own. She seems willing to trample anyone in order to get what she wants. She seems ready to kill. She would have no problem dancing on the backs of the bruised for the same reason stuck up rich people today don't care about the starving kids in Africa--she has never seen or experienced it. When reading Macbeth's letter that told of the witches' prophecy she said, "Yet do I fear thy nature; it is too full o' the milk of human kindness to catch the nearest way."
She wanted to take the quick and dirty route to royalty (murder), but didn't think Macbeth was up to it. She has never killed anyone (as far as we know), so she doesn't understand why it would be so difficult.
Macbeth is a battle-hardened soldier who is no stranger to danger. He has killed more than a few men, women, and children in his day. We're informed of that at the opening of the play when a sergeant tells of how Macbeth hunted down Macdonwald and "unseamed him from the nave to the chaps and fixed his head upon our battlements." But Macbeth knew the difference between right and wrong. Macdonwald was a traitor and fully deserved to be cut in half. Killing Duncan was a different story. Murdering a good king/friend in order to gain wealth...