Мусар к Берешит 4:8

The lesson we have to derive from all this is that if we do perchance commit a transgression, we should repent immediately and try to repair the spiritual damage such a transgression has inflicted upon us. Failing to do so brings about the great danger that one transgression leads to another, as indicated by the verse we have just quoted. Had Cain heeded G–d's warning to mend his ways (Genesis 4,7) he would not have become the first murderer in history. Having failed once to heed G–d's advice he not only murdered but turned heretic when he responded to G–d's question: "Where is your brother Abel," with the words "Am I my brother's keeper?" (Genesis 4,8-9), implying that there is neither judge nor justice in this world.