Infidelity

Seven men had been done in in their sleep- one for each night.

The logic to unravel this is quite similar to the logic behind the Three Hats. Consider what would happen if there had been only one unfaithful man. Every woman in town would immediately know, except that man's wife. But if you were that woman, wouldn't you quickly wonder why no gossip hound had informed of this fact? Of course you would- then it would dawn on you and CHOP! (or bang, or zap, or whatever) he's dead.

On day two, when there's no corpse, what would all the women think? "Aha! There's more than one philanderer in this town."

If there were exactly two unfaithful men in town, both their wives would only know about the other's infidelity. Combine this with the fact that they know there is more than one, "If there's two philanderers in this town, and I only know of one, and the only way I wouldn't know of the other is if he were..." Kablam!

And so it progresses. The wives of the faithful men believe (correctly) there to be as many unfaithful men as they know about, and no more. The wives of the unfaithful men (incorrectly) believe the same. As each night passes without a murder, the minimum number of cheaters rises by one.

As it happened, six nights passed uneventfully, meaning that there were at least six unfaithful husbands. The seven women who only knew about six spend that day buying their poisons, loading their weapons, sharpening their daggers and whatnot. And that night, The End.