Picture the scene: a suspicious husband returns home early and questions his wife's slave. The slave admits, under pressure, that her mistress is having an affair and the adulterous couple are upstairs at that moment. The enraged husband grabs his sword and rushes upstairs. He discovers his wife and her lover naked in bed and kills the man on the spot.
The man's family charge the husband with murder. There are no police in Athens, all prosecutions are carried out by private citizens.
In the ensuing court case, the wife testifies she saw her husband kill her lover.
The house slave testifies as to what happened and says she saw the killing. The slave is tortured as she speaks, as the law requires. A slave required to testify against her owner might lie to avoid later punishment; the immediate pain of torture is intended to overcome the fear of a later beating.
The neighbors testify too, because the suspcious husband had rounded them up as he returned home to act as witnesses when he suspected adultery. They had run upstairs after the husband and they too testify to seeing the husband kill his wife's lover.
The jury, having heard the eye witness reports, instantly dismisses the charges and the husband goes home a free man.
This really happened, in the early 4th century. The husband's name was Euphiletus. The dead lover was Eratosthenes (not the man of the same name who was Librarian at Alexandria a hundred years later). We don't know the name of the wife.
Adultery was illegal. If a man seduced a married woman then the penalty for the man was death. Since, as I said, there was no police force, it was perfectly acceptable for citizens to deliver DIY justice, as long as they were later found to be in the right. If the husband had dragged the body of the man into the street they might have got him for littering, but there was no way this was anything but a legal killing.
However if Euphiletus had not found the lovers in bed, but had later passed Eratosthenes in the street and punched him in the nose, then Euphiletus would have been the one in trouble for assault, because the burden of proof lies with the aggressor. Greek courts gave great weight to independent eyewitness reports, which is why Euphiletus rounded up his neighbors as he went home.
An adulteress could expect to be divorced, but it would have been illegal to kill her since she is not considered responsible for the adultery. Euphiletus has acted within the letter of the law, and the real criminal is dead.