Cain, the Whining Murderer.
Genesis 4:13 And Cain said unto the LORD, "My punishment is greater than I can bear."
Loosely translated, "Well, you don't have to have a cow."
Cain knew he was a murderer. Cain knew that he could not escape the punishment for his wickedness. So the only recourse left to him was to charge God with injustice.
The last resort of every ungodly person is to whine against God. Cain began by feeling sorry for himself because Abel's sacrifice was accepted and his rejected. He didn't have to offer a bad sacrifice, for we know from Abel's offering that God had given instructions to men on what to offer: the first of the flock with the fat thereof. Those who believed in the promise of mercy brought that sacrifice in faith [Heb. 11:4]. Cain had no faith, and therefore thought that anything he wanted to do would be all right. So his countenance fell and he moped around. God, seeing his bitchy attitude, told him that he could be accepted if he would shape up.
Instead, he went out to the field with Abel, quarreled with him, and murdered him. When God asked where Abel was [not to get information, but to call Cain to account], Cain responded with another whine: "Am I my brother's keeper," which was likely a sneer at Abel's occupation as a keeper of sheep. God did not deign to answer to whining question, because Cain had not been assigned to "keep" Abel. Cain was again making a whining charge against the injustice of God, as if he was assigned to the task of following Abel around to see that nothing bad would happen to him. There was probably history behind this whine, for Cain was the older brother and sometimes older brothers resent their younger brothers, especially if the younger one is more favored of God. No Cain, you are not your brother's keeper, but that doesn't mean you get to kill him.
So God, knowing the wickedness of Cain murderous heart, proceeded at once to the sentence: "What hast thou done? the voice of thy brother’s blood crieth unto me from the ground." Cain was deprived of all the blessings of the earth and sentenced to be a vagabond and a fugitive, unable to farm the field as he had before. God, having compassion, put a mark on Cain so no one would kill him.
But whiners, feeling sorry for themselves, never quit whining. The man with one talent, hid it in the ground and complained about God's injustice [Matt. 25:24]. In the same way, Cain continued to complain about God, about how unfair He is: "My punishment is greater than I can bear. Poor me, poor, poor Cain. God is so cruel." Still not a shred of sympathy for the man he murdered. Not a shred of feeling for his family, his mother, his father, and what very likely was a large family by then. Just poor, poor me.
Don't feel sorry for Cain. He didn't get nearly what he deserved. None of us do--in this life.