Because he had eaten Poseidon and Hades to stop any of his children from taking his throne.
Addition:
Since Cronus was divine he could not be killed, but they sent him into the underworld.
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Zeus had many siblings: Poseidon, Hades and Hera were some of them. They all fought against they're father, Zeus was the one to kill him and spread his body parts around. Since he killed his father, Zeus became the King of the gods, he tricked Hades into taking the underworld, he gave the whole sea to Poseidon and he married Hera his sister.
No Zeus did not kill Poseidon in Greek myth.
In Greek myth, Hades never attempted to kill his siblings and nephews/ nieces the Olympian gods and goddesses. He did fight beside his brother Zeus and Poseidon against the Titans.
Zeus, god of the sky, didn't actually kill his father because his father (Cronus) was immortal, like Zeus.Instead, Zeus used his fathers own weapon to slice Cronus into a thousand pieces. Zeus then tossed all the pieces into Tartarus, the deepest crater in the underworld, and Cronus never escaped. Cronus was still alive because he was immortal, but he was harmless since his body was scattered in Tartarus.Zeus was then chosen to be king of the gods. Him and his brothers, Poseidon and Hades, chose the domains they would rule over. Zeus picked the sky, Poseidon picked the sea and Hades picked the underworld.Zeus did not kill Cronus, he and his allies fought a war and Cronus and the other Titans were driven into Tartarus.
because zeus sent haydes into the underworld..so for revenge he tried to kill zeus son..