This painting depicts the the slaying of Abel by his jealous brother Cain as described in the Book of Genesis. God favored Abel's sacrifice more than Cain's, and the sin of Cain's burning envy drove him to kill his brother.

God's response to Cain's jealously is shown in the detail below, "Sin is a demon lurking at the door; his urge is toward you, but you can be his master." (Gen. 4:7) Cain failed in accepting this wisdom and allowed sin to master him.

The design of the figures is loosely based on a marble sculpture adorning the outside of Milan Cathedral.

 
Detail
"Sin is a demon lurking at the door..."
 
"...his urge is toward you, but you can be his master. "