The first murder was Cain's murder of his brother Abel according to the Old Testament.
Now Cain was a bad dude, in fact he was bad to the bone. As you recall, Cain and his brothers were farmers, Abel was the livestock guy, and Cain the tiller of land. The murder comes about when Cain gets attitudinous when God shows some real appreciation for the offering made by Abel, and less than genuine pleasure at a half-assed offering by Cain. In fact God warns Cain, "sin is crouching at your door." This caution doesn't even cause Cain a momentary pause. In a premeditated act Cain suggests to Abel that the two of them take a walk out into the field where Cain promptly spills his brother's blood. In the OT account, God crossexamines Cain, "where's your brother?" What does this wise guy say to the Creator? Uh, like dude "am I my brother's keeper?"
Holy mackeral, what a wise ass! Let's just say that the fact that Cain was not instantly a small smolddering pile of carbon, should tell us all something about mercy. In fact the first murderer is not only not killed, he is given a mark to protect him against being killed. This murderer moves east of eden and builds a city in the land of Nod.
Look noone is going to miss Lawrence Reynolds (pic) was 43 years old. He was a complete loser; he killed his 67 year old neighbor while fully addled on drugs. He stole $40. Still, Reynolds, was considering the state of his impairment at the time he committed his pathetic crime, less culpable than Cain. Nonetheless, Ohio showed no mercy.