Genesis 4:8

And Cain talked with Abel, his brother. The warning of the Lord was disregarded, deliberately set aside, as he started a quarrel with his brother. And it came to pass, when they were in the field, that Cain rose up against Abel, his brother, and slew him. Cain did not try to keep the sinful desire of his heart in subjection, and so the end of the quarrel was murder. Note that the words "his brother" are repeated again and again, in order to emphasize the heinousness of the first murder. In our hearts also evil thoughts are found: murders, with all the jealousy, envy, bitterness, hatred, and anger that this climax of wickedness presupposes, and our constant endeavor must be to conquer the inclination toward all these sins and to keep the example of pious Abel before our eyes.