Cain did not offer his sacrifice in faith.  He offered vegetables instead of the first born lamb.    

We should always look out for each other.

Cain offered fruit from the ground to offer to the Lord.  Cain killed his brother.  Cain had no faith.  Cain lied to the Lord.  Cain did not believe in the Lord.  Cain loved Satan more than God.  Cain was wroth with the Lord.  Cain glorified in his wickedness.  Cain followed Satan. 

We always have agency to choose what to do.  You cannot serve both God and Satan.  We have the freedom to choose eternal life or captivity and death.  We will be rewarded according to our works.