The Scarlet Blossom by Vsevolod Garshin

He did not sleep all night. He had plucked the flower because he saw in this action a deed he was in duty bound to perform. At the very first glance through the glass door the blood-red petals had attracted his attention, and it seemed to him that from this moment it was perfectly clear what in particular he was called upon to perform on earth. In this brilliant red flower was collected all the evil existent on earth. He knew that opium is made from poppies, and perhaps this knowledge, taking some fantastic, distorted form, had induced him to create this terrible and monstrous phantom. In his eyes the flower was the personification of all evil. It flourished on all innocent bloodshed (which was why it was so red), on all tears, and all human venom. It was a mysterious, awful being, the antithesis of God -- Ahriman -- who had taken a modest and innocent form. It was necessary to pluck and kill it. But more than this was necessary; it was necessary not to allow it to emit all its evil into the world. Therefore he had hid it in his chest. He hoped that by the morning it would have lost all its strength, that its evil would have passed into his body, his soul, and there be conquered or conquer -- when, if the latter, he would himself perish, die, but die as an honourable knight and the first to wrestle at once with all the evil in this world. "They have not seen it. I saw it. Could I let it live? Better death!"

And he lay wearing himself out in a struggle, phantom and unreal, but nevertheless exhausting. In the morning the "dresser" found him scarcely alive. But this notwithstanding, in a short time excitability once more gained the upper hand. He jumped up from his bed, and resumed his former race through the passages of the Asylum, conversing with the other patients and himself more loudly and disjointedly than at any previous time.