(Before I start, Greek mythology is my element)
Okay, so Hades was part of the Big Three, including Poseidon, Zeus, and himself. After killing Kronos, Hades was sent to take charge of the underworld. He hated it. HE IS NOT THE GRIM REAPER, he simply governs over the dead, and makes sure the dead souls stay where they are and not intrude in other areas of the underworld (Elysium, Running Fields, and the pits of Tartarus).