The body of myth involving Hercules includes stories where Hercules commits an offense without having to pay serious consequences, but this isn't one of them. As a result of the killing of his guest Iphitus, Hercules is sold on the slave market to the Queen of Lydia, Omphale. Here we will look at the sequence of events that led Hercules to such sacrilege.
Iphitus was one of the sons of famed archer, King Eurytos (Εὔρυτος) of Oechalia (Οἰχαλία, Oichalia, Oikhalia) and one of the brothers of the beautiful Iole. Iphitus passed on his father's bow, a gift from great-grandfather Apollo, to Odysseus, who later used it to slaughter the suitors.
Apollo had taught Eurytos to shoot, and Eurytos may have taught Hercules to shoot. One version of how Eurytos met his end is that he challenged Apollo to an archery contest, an act of hubris too great to tolerate. Apollo killed him. Another version, and the one that directly involves Hercules, has Eurytos offer a challenge: whoever beats him in archery will get to marry his beautiful daughter Iole.
Hercules was up to the challenge, accepted, and won. Eurytos reneged, which is not a wise thing to do to either a god or Hercules. One version has the sons disarm and beat up Hercules at a banquet, before flinging him from the palace.
Hercules did not attack Oechalia immediately, but he did arrange the theft of the mares (Apollodorus: cattle) of Eurytos, either alone, or with the skill set of Autolycus.
Eurytos' son Iphitus set out to find the stolen mares. He visited Hercules, hoping the hero would help in his quest, but Hercules had other, temporarily mad thoughts. It is worth noting that one reason given for Eurytos' refusal to surrender Iole is fear that Hercules would go mad, again, and kill her, so Hercules' sacrilege and later actions, show that Eurytos may have had good reason. While Iphitus was his guest, Hercules took him to see the sights around the Tiryns palace, and then pushed him from behind to his death from the top walls. It doesn't matter that the sons of Eurytos had been less than hospitable towards him.
Disposing of one of Eurytos' boys was not the end of Hercules' revenge. He later took an army to Oechalia, sacked the city, killed Eurytos, and carried off Iole as concubine, but first he had to pay for his violation of the guest-host relationship. Sold to the Lydian queen, he spent perhaps three years at her court, where at least some of the time, he dressed in women's clothing.
Main Source: The Heroes of the Greeks, by C. Kerenyi
The story of Iphitus in one form or another appears in the Odyssey XXI, Sophocles Trachiniae 270-73, and Apollodorus (II.VI), as well as Diodorus Siculus IV.21
