Zeus sends Hermes, the messenger-god, because he's the only one who regularly goes there on business (Hermes guides souls to Tartarus).
Zeus sent the god Hermes to intervene in the myth of Demeter and Persephone. Hermes acted as a messenger between Hades and Demeter, helping to negotiate Persephone's release from the Underworld.
Hades fell in love with Persephone when he saw her picking flowers in a meadow and decided to abduct her to the underworld to be his queen. Persephone's mother, Demeter, was devastated by her daughter's disappearance and persuaded Zeus to bring her back, but because Persephone had eaten pomegranate seeds in the underworld, she had to spend part of the year there, leading to the changing of the seasons.
Zeus did not "send Hades to the Underworld"; just as Zeus rules the heavens, Hades rules the Underworld. According to one of the myths, Zeus selected the Heavens as his realm, and allowed Poseidon to select next because he was eldest. That left Hades (or more accurately Plouton) the Underworld as his realm, which he found suited him just fine.
Demeter, her mother, demanded it: or no living thing would grow. Now half the time Persephone is in the Underworld it is winter, and when she returns it is spring.
Zeus, Hades, and Poseidon, or the Big Three, swore on the River Styx, to no longer have demigod children, so when Zeus had Thalia, he broke his promise. Hades, angered that he broke his promise, sent horrible monsters after her.
Persephone, being Queen of the Underworld and the dead, did not kill - but she could send plagues as she and Hades did upon Thebes when they would not bury the dead warriors after the Seven Against Thebes war.
Hermes leads the dead down into Hades.
It wasn't Zeus. It was God.
Epimetheus
Hades sent the Minotaur to abduct Percy's mother.
To the Underworld, and levels therein.
no