So who was the murderer again...

The murderer is never revealed: a mystery without resolution. The final scene suggests that everyone was in on the same conspiracy, but even this is open to question.

SPOILERS

The implication is that Mrs Herbert and her daughter, Mrs Talmann, were the ones who planned the murder and the execution.

Mrs Herbert's gardener, Mr Clarke, murdered her husband the day he returned, which is on the 12th day at the 13th site. Mrs Herbert and her daughter then framed the draughtsman as the murderer (after ensuring Mrs Talmann's pregnancy). They also manipulated the other men into becoming judges and executioners.

The clues in the draughtsman's drawings are the signs of what's to come, not what had already happened. That's why Mrs Talmann scorned the draughtsman's lack of attention to details, even though he had paid attention to them. He noticed the details, but not in a way he thought them to be. He was too arrogant and self-absorbed to see them more than what they were. In other words, he failed to recognise those clues as warnings. Those are gestures of intelligence and a malicious sense of humour on the women's part.

Like, "You think you're so clever, sir. We laid out all these clues for you to recognise and run away, but you failed to heed all these warnings. You're a silly fool after all. Just like all other men around us."