Why Did Kratos Kill Baldur?

Kratos killed Baldur to save Freya, Baldur's own mother, whom Baldur was strangling to death. Kratos had beaten him and let him live only moments earlier, on the condition that he leave his mother alone. When Baldur went straight for her anyway, Kratos broke his neck, saying the cycle had to end. (God of War (2018))

Why Baldur was so hard to kill#

Baldur spends most of God of War (2018) hunting Kratos and Atreus, and nothing hurts him. The reason is his mother. Freya had seen that her son would die, and to stop it she cast a spell that made him immune to every threat except mistletoe. (God of War (2018))

The protection had a cost. It took away all feeling, so Baldur couldn't feel pain, pleasure or even the cold. After about a hundred years of that, he hated his mother and wanted her dead. The page on Baldur's curse covers the spell in full. (God of War (2018))

The last fight#

Near the end of the game, Kratos, Atreus and Freya come out near the frozen corpse of the giant Thamur, and Baldur is waiting. It's the first time in years that he and his mother have met, and he attacks. (God of War (2018))

When Atreus steps in to protect his father, Baldur hits the boy in the chest. The blood is Baldur's. Earlier in the game Kratos had fixed a broken strap on Atreus's quiver, and one of the arrows carried mistletoe. It went through Baldur's hand, and the spell broke. For the first time in a century he could feel, and he could die. (God of War (2018))

Kratos and Atreus beat him. Baldur dared Kratos to finish it, but Atreus and Freya begged him not to, and Kratos spared him. He warned Baldur not to come after them again and not to lay a hand on his mother. (God of War (2018))

Why Kratos broke his neck#

Baldur ignored him. He went for Freya and started to choke her, and she let him, as a way of showing him how sorry she was. Kratos stepped in and snapped Baldur's neck. (God of War (2018))

So the plain answer is that Kratos killed Baldur to save Freya's life, after giving Baldur a chance to walk away. But Kratos also says why it matters to him: the cycle has to end here. (God of War (2018))

That cycle is the story of his own family. In Greek myth and in God of War, Cronos was overthrown by his son Zeus, and Zeus feared the same from his own son, Kratos, which is why he turned on him. The page on the cycle of patricide traces it. Here, for once, a son is about to kill a parent, and Kratos steps in on the parent's side. (God of War II; God of War (2018))

Our reading, and it's an interpretation, is that this is also about Atreus. Kratos spends the whole game afraid his son will turn into what he was. Killing Baldur in front of him, to save a mother from her child, is Kratos trying to stop the pattern before it reaches the next generation. (God of War (2018))

What it cost#

Freya doesn't thank him. She holds her son's body, swears she will have revenge on Kratos, and disappears with Baldur. (God of War (2018))

Mimir later points out the bigger problem. Baldur's death is the first sign of Ragnarök, and it has started Fimbulwinter, a winter that will last three years. The Aesir had thought the end was a century away. (God of War (2018))

The fallout fills the sequel. Freya hunts Kratos through the winter until, in Ragnarök, the two make peace over her son's death. The full scene is on the page for Baldur's death, and the rest of the story is in the Ragnarök ending explained. (God of War (2018); God of War Ragnarök)