Why did Cain kill Abel?What does the text mean when it says, "Sin is lurking at the door; its desire is for you, but you must master it?"

In Genesis 4, Cain is Abel's older brother.  He is put in
charge of the fields ("fruits of the soil") and Abel is put in charge of the
flocks.

It says in the text that while Cain brought "some
of the fruits of the soil" as an offering to the Lord, his brother, Abel, "brought fat
portions from some of the firstborn of his flock."  (Genesis 4, NIV)  Notice the
difference in the offerings.  Cain's is just an offering of something, he doesn't even
consider giving the Lord the best of the fruit, let alone the
first of the harvest.  Abel does both.  He gives the fattest of the
firstborn animals.

Cain's afterthought of a sacrifice shows
that he is not honoring God before everything else.  He is acting
selfishly.

Because of this, the Lord looks favorably on
Abel and therefore Cain is jealous.  When God says, "Sin is
lurking at the door; its desire is for you, but you must master it," he is telling Cain,
You have the opporunity to change your ways and win my favor - but you must
overcome the temptation to be jealous of your brother.  You must also offer me a
sacrifice that comes from your heart and shows me how much you love me more than
anything else in this world...

Instead of
heeding this advice, Cain kills his brother in jealousy and
rage.