In his ambition and drive to become king, Macbeth seeks to eliminate all who stand in his way. In Act I, Scene IV, Macbeth speaks with Duncan, the king of Scotland, who mentions that he has named his son Malcolm as heir to the throne. Hearing this, Macbeth exclaims:
Macbeth: [Aside.] The Prince of Cumberland!
I must fall down on that Step, or jump over it,
For it’s in my way. Stars, put out your light!
Don’t let anyone see my black and deep desires.
He has already resolved to murder Duncan and thus ascend to the throne, but Duncan has placed an obstacle in his way; his own son. Macbeth, realizing that he will have more work to do to gain the throne, prays that his evil thoughts will be hidden from those around him. The use of "step" to describe Malcolm indicates Macbeth's disdain for human life; Malcolm is merely an obstacle to be jumped (killed) or he will "trip up" Macbeth in his quest for kingship. He also recognizes his desires as "black," or evil, but his ambition allows him to ignore this recognition.