Why Did Kratos Kill Helios?

Kratos killed Helios because the sun god fought for Zeus in God of War III, lied to him about how to beat Zeus, and stood between Kratos and his revenge. There was also a practical reason. Helios's head gave off the light of the sun, and Kratos tore it off and carried it with him as a lamp and a weapon. (God of War III)

The debt from Chains of Olympus#

Kratos and Helios had history. In Chains of Olympus, the goddess Persephone had the Titan Atlas tear Helios out of the sky as part of her plan to destroy the world. With the sun gone, the dream god Morpheus put the gods to sleep. Kratos defeated Persephone, chained Atlas under the world and returned Helios to the heavens. Afterwards Helios felt pity for what Kratos had given up, but Athena refused him any more help. (God of War: Chains of Olympus)

So when the two met again, Helios had reason to think Kratos might spare him. (God of War: Chains of Olympus; God of War III)

Helios on the wrong side#

By God of War III Kratos has turned on Olympus. He climbs the mountain with the Titans, and the gods come out to fight. Helios takes to the sky in his chariot, throws fire at Gaia and battles the Titan Perses on the slopes. (God of War III)

Kratos uses a ballista to damage the sun chariot, and it flies straight into Perses's hand. The Titan crushes Helios and throws him far into the city. Kratos finds him there, too badly hurt to stand, guarded by soldiers with shields that Kratos breaks open with a captured Cyclops. (God of War III)

The lie about the Flame#

Helios begs for his life. He reminds Kratos that Kratos once saved him and promises to repay the favour. Kratos only wants to know where to find the Flame of Olympus, the power he needs to get at Zeus. Helios tells him he will never defeat Zeus and will die trying. Kratos answers that his own life is not the one Helios should be worrying about. (God of War III)

Helios then tries to blind him with the light of the sun. Kratos shields his eyes, closes in and stamps on his head until Helios begs him to stop. Helios tells him he must bathe in the Flame to take its power. Kratos knows that's a lie, because the smith god Hephaestus has told him the Flame kills anyone who touches it, and he trusts Hephaestus because Hephaestus has nothing to gain by lying. Helios insists that killing him won't lead Kratos to Zeus. Kratos tells him he's wrong, breaks his neck and tears his head from his body. (God of War III)

Why the head mattered#

That last exchange is the clearest answer the game gives. Kratos didn't kill Helios only out of hatred. He needed the light. He carries the Head of Helios for the rest of the game, holding it up to reveal hidden passages, light dark places and blind enemies. He even uses it twice to blind the Titan Cronos. (God of War III)

Our reading is that the scene shows how far Kratos had gone by then. Helios was not one of the gods who had wronged him most, and Kratos had once gone out of his way to save him. Kratos killed him anyway, because he was in the way and because his head was useful. (God of War: Chains of Olympus; God of War III)

What it cost Greece#

Each god Kratos killed in God of War III took something from the world. Poseidon's death flooded the land. When Helios died, clouds swallowed the sun and darkness, rain and storms swept across Greece. Kratos didn't care at the time. (God of War III)

He did later. In the Valhalla epilogue to Ragnarök, an illusion of Helios's severed head takes Mimir's place at Kratos's side and mocks him over what he did in Greece. It is one of the memories Kratos has to face before he can move on. The Valhalla story explains how those trials work. (God of War Ragnarök: Valhalla)

For more on Kratos's war with Olympus, see why did Kratos kill Zeus and the God of War III ending explained. (God of War III)