The adventure of the devil's foot by Arthur Conan Doyle

 Another short story. 

For whatever reason, Holmes suggests writing about the Cornish horror case, various years after it happened.

Watson and Holmes had left London in search of peace and quiet, but when there’s a mystery there they are of course called upon to solve it, though not by the police but simply by the local vicar.

A man, Tregennis, says that he had visited his two brothers and his sister the night before, and this morning they were found sitting just like he had left them, only the sister was dead, apparently by fear, and the brothers were out of their sane minds and had to be taken away - I have no idea if they will regain full control of their minds or not, the story doesn’t go that far.

Dr Sterndale, a lion hunter who spends much of his time in Africa, was supposed to leave that morning, but stayed behind as soon as the vicar sent him a telegram with what happened.

Tregennis is later found dead in the same way, in his rooms, scared to death.

Holmes finds a weird powder and tries the experiment himself, lighting a lamp with the powder in, with nearly deadly consequences was it not for Watson who hurried them both out of the house.

Holmes has all the story, but questions Sterndale to learn his motives and decide what to do.

The thing is, Sterndale loved the Tregennis sister, but he couldn’t do anything about it because he was married, although separated, and the law did not permit him to get a divorce. Still, he loved her very much and she loved him. He had met her three brothers and had told Tregennis of that powder among a variety of bizarre things from Africa. He explained in great detail, answering all the other man’s question, with no suspicion (sure, let’s speak of this undetectable, unknown mean of killing people, why not, what could possibly be wrong with that…)

When he heard what happened, he was sure Tregennis did it, so he confronted him and held him at gunpoint leaving him in a locked room with the lighted lamp with powder, until he died.

Knowing now that the man acted out of personal justice, a vendetta for love, or however you want to put it, Holmes lets him go back to Africa. After all, the police never contacted him, so he had no obligations towards them.

(The reason why the sister died while her brothers only went mad, is thus explained: the powder was put in the chimney, so the effect was not as strong, since “the window was shut, but the fire would naturally carry fumes to some extent up the chimney […] only the woman, who had presumably the more sensitive organism, was killed, the others exhibiting that temporary or permanent lunacy which is evidently the first effect of the drug”