It happened in what would be Spain, about 430,000 years ago. One early neanderthal picked up a rock and slammed it into the face of another. The victim bleated and swayed, so the attacker brought the hand axe down again. The other collapsed, quivered, and died. The victim ended up at the bottom of a deep natural chimney, to be buried in silt and preserved. What became of the aggressor can never be known, but if history is any guide the answer is likely, “Nothing.”
It probably wasn’t the first murder but It absolutely, certainly was not the last.