Macbeth, a seasoned warrior (who, in a recent battle, cut the traitor Macdonwald in half), comes back to his chambers badly shaken from murdering his monarch. Seized by a deep revulsion at his act, he says to Lady Macbeth:

Will all great Neptune's ocean wash this blood
Clean from my hand? No, this my hand will rather
The multitudinous seas incarnadine,
Making the green one red.

He is using hyperbole (exaggeration) in this speech to try to convey to his wife the depth of his horror at what he has done. It is one thing to kill a traitor in battle but quite another to kill your king to whom you have sworn your loyalty.

Neptune is the god and ruler of all the oceans on earth. Macbeth says that all of Neptune's vast oceans cannot wash the blood of this murder from his hands. Instead, still using hyperbole, Macbeth says that his bloody hand will turn the many green seas of the earth red ("incarnadine").

This speech expresses the depth of Macbeth's horror, but it also emphasizes Macbeth's conviction that, in murdering a king, he has gone against nature itself. What he has done is so unnatural, so heinous, it could change nature's very colors from green to blood red.

We know that Macbeth, a brave warrior, has no problem with killing an enemy on a battlefield. His speech shows, however, that he puts regicide (king slaying) in a whole different category—the worst kind of crime.