Who among you is wise and understanding? Let him show by his good behavior his deeds in the gentleness of wisdom.  But  if you have bitter jealousy and selfish ambition in your heart, do not be arrogant and so lie against the truth .  This  wisdom is not that which comes down from above, but is earthly,  natural , demonic.  For  where  jealousy and selfish ambition  exist, there is disorder and every evil thing.  But  the wisdom from above is first pure, then peaceable, gentle, reasonable, full of mercy and  good fruits , unwavering, without hypocrisy.  And the seed whose fruit is righteousness is sown in peace by those who make peace .  ( Ja 3:13-18 )

Kain  did not sow pure seeds; therefore he reaped earthly, natural, demonic fruits, which were rejected on the altar.  His bitter jealousy led to his murder/manslaughter of Abel in the open field. He cut down a "fruit tree" because of selfish ambition to be respected at the altar...his "synagogue."  He wanted Abel at his "footstool."  There is an unfinished sentence describing Abel's death:

Cain  told Abel his brother...  And it came about when they were in the field, that Cain rose up against Abel his brother and killed him. (Ge 4:8)

Kain told his brother what?  The text doesn't say!  It was an unfinished argument, perhaps a slander left hanging and uncorrected in the company of their parents or brothers and sisters, then ending later in the field without human witnesses. No one made peace between brothers, so there was a general fruit failure.

Religious ambition is DANGEROUS!  It will lead eventually to bloodguiltiness and wandering from city to city, never finding a resting place.  Economic ambition can also lead to death:

Come now, you who say, " Today or tomorrow we will go to such and such a city, and spend a year there and engage in business and make a profit."   Yet  you do not know what your life will be like tomorrow.  You are just a vapor that appears for a little while and then vanishes away.  Instead , you ought to say, "If the Lord wills, we will live and also do this or that."  But  as it is, you boast in your arrogance; all such boasting is evil.  Therefore, to one who knows the right thing to do and does not do it, to him it is sin . (Ja  4:13-17 )

James references the Torah portion, for if a person knew he would be murdered between cities, he would not boast of how much money he'd make in the next city. He references the guilt of the stiff-necked, applying it to the personal decision-maker's bragging tongue, not only the judges and elders.  A person who draws excessive attention to himself will attract predators.  Arrogance can lead to death, making one disappear like a vapor, which is what Abel (hevel) means.

Is it possible that Kain was wandering from the straight path long before he killed his brother?

My brethren, if any among you  strays  from the truth and one turns him back ,  let him know that he who turns a sinner from the error of his way will save his soul from death and will cover a multitude of  sins.  ( Ja  5:19-20 )

An official of the post-Edenic community should have spoken up.  It is not likely that Kain and Abel brought their sacrifices alone.  They would have celebrated firstfruits with their family.  Adam and Eve should have spoken up, counseled their sons before they went into the field alone, but perhaps once again, Adam remains silent when an impending war threatens to chop down a fruit-producing tree, a human being made in the image of Elohim, a human being who will wander away from the Tree of Life and one whose blood will cry from the ground.

So who was the eglah arufah?

* The stiff-necked elders and judges who 

a)  looked the other way while crimes were committed?  

b)  took the bribes of favor to rule unfairly?

* The sinful wanderer with no one to turn him back when he strayed from the path onto untilled soil?

* The traveler who boasted of his riches but couldn't carry them to the real "tomorrow"?

* The righteous victim of envious, bitter human predators?*

Maybe the broken-necked calf represents all of these.  Our public officials must bear the guilt of looking the other way when crimes are committed or obstructing justice.  The difficulty today is that our communities are typically so populous that finding the closest officials to hold responsible is a challenge.  The number of murders exceed the ability of officials to investigate and find the murderers, which is why this practice was abandoned in the Second Temple era. The number of murders made it impossible.

What can the Israelite lay-judge do?  Simply put, don't turn our necks to crime. Become involved in the election of public officials.  Identify candidates who seek justice, not a higher position.  The more humble the candidate for position or office, the more likely he or she is to be resistant to bribes of favor or to turn his or her neck to crime.  Write letters to your representatives.  Email them.  Show up for town halls.  Vote.  Run for office.  Encourage Spirit-filled young people to seek careers in law enforcement, law, or government.  Urge justice for those who have no voice.  Feed and clothe the poor.  Be salt and light to your closest neighbors.  Make peace.  Forget the silly political posts on social media and GO to places where your zeal for righteousness will stop broken necks before they start. Kudos to Dr. Dinah Dye for becoming involved in local politics and trying to restore justice in her community.

Life and death are in the hand of the tongue.  We are all judges, and we will eat the fruit that our tongues pluck.  Choose life.

*According to tradition, Miriam's son Hur was murdered by the idol-worshipers when he spoke up against the Golden Calf.