This story has a number of twists and turns. You have a hero who is hated by his step-mother, yet she confers immortality on him. You have her then driving him mad and killing his own family, resulting in the renowned 12 labors. As one of the early labors, Hercules acquires a deadly poison, but the labor doesn't count because Hercules had help in performing it. Adding injury to insult, part of the bounty of that labor led to Hercules' own demise.... Read on and then make sure to follow the hyperlink below to read about Hercules' apotheosis:
When Hercules was trying to take his bride Deianeira home, he had to cross the Evenus River. Nessus, a centaur, served as ferryman. First he rowed Hercules across and then, as he started to row Deianeira across, he tried to rape her. Hercules, justly enraged, drew one of his poisoned arrows [see Hercules Labor 2] and shot the centaur. Before he died, the centaur persuaded Deinaeira to take some of his blood to use as a love potion should Hercules ever cause her to worry.
In time, Deianeira became suspicious of Hercules' interest in another woman, named Iole, so she smeared some of the carefully-saved centaur blood on a tunic and gave it to Hercules, trusting that it would act as a love potion and return him to her.
Unfortunately, the centaur had lied. The blood contained not a love potion, but a powerful toxin from the poison with which Hercules had tipped his arrows. It had come from the Lernaean hydra that the hero had killed in his second labor.
When Hercules put on the tunic, it burned his skin. He was in such excruciating pain that he wanted to die. Note that the burning would have killed an ordinary human, but Hercules was not such a one [see Apotheosis of Hercules]. After consulting an oracle for advice, he had a funeral pyre built for himself. He then mounted it and eventually persuaded a friend to light it. He was then allowed to die and went to the gods where he was reconciled with his tormenter, the queen of the gods, Hera. She allowed him to marry her daughter Hebe and live among the gods thereafter.
Intending to offer sacrifice, [Hercules] sent the herald Lichas to Trachis to fetch fine raiment. From him Deianira learned about Iole, and fearing that Hercules might love that damsel more than herself, she supposed that the spilt blood of Nessus was in truth a love-charm, and with it she smeared the tunic. So Hercules put it on and proceeded to offer sacrifice. But no sooner was the tunic warmed than the poison of the hydra began to corrode his skin; and on that he lifted Lichas by the feet, hurled him down from the headland, and tore off the tunic, which clung to his body, so that his flesh was torn away with it. In such a sad plight he was carried on shipboard to Trachis: and Deianira, on learning what had happened, hanged herself. But Hercules, after charging Hyllus his elder son by Deianira, to marry Iole when he came of age, proceeded to Mount Oeta, in the Trachinian territory, and there constructed a pyre, mounted it, and gave orders to kindle it. When no one would do so, Poeas, passing by to look for his flocks, set a light to it. On him Hercules bestowed his bow. While the pyre was burning, it is said that a cloud passed under Hercules and with a peal of thunder wafted him up to heaven.
Apollodorus Lib. 2.7.7