Why Did Kratos Kill His Family?

Kratos killed his family by accident. Ares, the god he served, secretly moved his wife Lysandra and daughter Calliope into a temple in a village Kratos was raiding, and Kratos cut down everyone inside before he saw who they were. Ares did it on purpose, to cut Kratos's last ties and make him the perfect weapon. (God of War (2005))

How Kratos came to serve Ares#

The first game tells Kratos's past in flashbacks. He was a Spartan captain who wanted glory more than anything. When his army was about to be wiped out by a barbarian horde led by Alrik, he called on Ares and offered his life in exchange for victory. Ares answered. He destroyed the barbarians and bound the Blades of Chaos to Kratos's arms as a mark of his service. (God of War (2005))

From then on Kratos led the god's campaigns. He raided and burned his way across Greece in Ares's name, and the flashbacks make clear he did it willingly. (God of War (2005))

The night at the temple#

One night Kratos led an attack on a village whose people worshipped Athena. He burst into her temple and killed everyone he found inside. Only when it was over did he see that two of the dead were Lysandra and Calliope, whom he had left at home in Sparta. (God of War (2005))

He hadn't known they were there because he wasn't meant to. Ares had moved them to the village and placed them in the temple so that Kratos would be the one to kill them. (God of War (2005))

The village oracle then cursed him. The ashes of his wife and daughter were bound to his skin for good, a mark of what he had done that anyone could see. That curse is the answer to why Kratos is white, and it gave him the name the Ghost of Sparta. (God of War (2005))

Why Ares did it#

The game spells out the god's reason in its final fight. Beaten down by Kratos, Ares reminds him that he saved his life against the barbarians, and says that making him kill his family was meant to turn him into the perfect warrior, with nothing left to hold him back. Kratos tells him he succeeded, and runs him through with the Blade of the Gods. (God of War (2005))

So the deaths weren't a random cruelty. Ares saw Lysandra and Calliope as the last human part of his servant and wanted it gone. It worked in a way he didn't expect: the warrior he made turned on him. (God of War (2005))

What happened next#

Kratos renounced Ares. Breaking a blood oath to a god wasn't simple, and God of War: Ascension shows the Furies hunting him for it until he kills them and frees himself. (God of War: Ascension)

He then served the other gods for ten years, hoping they would take away his nightmares of that night. They never did. In the first game's story, Athena promises that the gods will forgive him if he kills Ares. Even in that last battle, Ares traps him in a vision where he must defend his family, then kills them again in front of him. (God of War (2005))

The memory followed him for the rest of the Greek saga. In God of War III, close to death in the final fight with Zeus, Kratos meets the spirits of Lysandra and Calliope inside his own mind and finally forgives himself. (God of War III)

Was it really his fault?#

That's the question the series keeps asking, and the answer it gives is both. Ares set the trap, but the trap only worked because of who Kratos was by then. He was killing strangers in a temple without a second thought, and his family died because they were standing among them. (God of War (2005))

Much later, in the Valhalla epilogue, Kratos looks back on his Spartan years and admits he was a poor husband to Lysandra. Our reading, and it's only a reading, is that this is the series' settled view: Ares is to blame for the trick, and Kratos accepts the blame for the man who fell for it. (God of War Ragnarök: Valhalla)

For more on the family Kratos lost and the one he built later, see Kratos's wives. (God of War (2005); God of War Ragnarök: Valhalla)