As is often the case in Greek mythology, there are various versions of the myth of Sisyphus. It is generally agreed that he was punished for offending Zeus, and that one of the reasons for this is that he murdered guests in the palace of Ephyra, where he was the king. Zeus, as well as being king of the gods, was the patron of "xenia" or "guest-friendship," meaning that any offense against a guest was particularly hateful to him.
The specific incident which made Zeus devise such a cruel punishment for Sisyphus, rather than simply consigning him to Tartarus, occurred after Sisyphus's death. Zeus commanded Hades, the god of the underworld (or, in some versions, Thanatos, the spirit of death) to chain Sisyphus down. Sisyphus asked Hades to demonstrate the mechanism of the chains and, catching him unawares, escaped and chained down his former captor in his place.
While Hades was chained down, no one could die. This meant that the old and sick had no release from their pain, while Ares, the war god, complained that battle had become a travesty without the possibility of death. It was for this piece of trickery that Zeus is supposed to have doomed Sisyphus to rolling the stone up the hill forever.