God of War Source

Since people ask about what happens at the end and why, I will explain it to you:

It all began when Kratos first opened Pandoras Box back in God of War I. The Evils locked inside were released. Instead of simply taking over the world again they infected the gods and changed them. This ist he reason why the gods appeared to be more evil in God of War II. Each of them was infected with one of the Evils from Pandoras Box. Zeus was infected with Fear. This Evil drove Zeus into killing Kratos, he was afraid his son would turn against him, just like Zeus himself had turned against Kronos. When the Box was created, Athena feared what could happen i fit was opened ever again. So she summoned Hope and placed it in the box as well. This ist he power Kratos used to kill Ares. But after the gods refused to take /away his nightmares Hope became locked by Kratos guilt and thoughts of failure. And so he wasnt able to use it again.
However, nothing of this was aware to ANYONE. Athena thought Kratos absorbed the Evils from Pandoras Box to slay Ares, not knowing that the gods were infected with these „bad powers. That is why she told Kratos to find Pandoras Box in the beginning of God of War III. She wanted Kratos to open the Box and take Hope to defeat Zeus. But since Kratos already took this power, the Box was empty when he opened it after Pandoras sacrifice. After Kratos was attacked by the spirit of Zeus, he learned to forgive himself and forget about his failures and so after all those years he was finally able to unleash Hope and use it to kill Zeus.

Now… What about Athena?
It is possible that she was infected with the Evil Greed. This could explain why she desired the power of Hope so badly and rebuild the world as the only goddess. The only thing that makes no sense is that whenever Kratos killed a god, the Evil he was infected with causes a plague. (Poseidon: Floot / Hades: The sould of the dead enter the world / Helios: The sun vanishes / Hermes: Flies cause diseases and death / Hera: Plant die off / Zeus: Thunder and storms appear) When he killed Athena, however, Greed would escape her as well. But as you can clearly see Athena is still greedy after her death. I uses the quotation mark because Athena still is somehow alive. This may be because she sacrificed herself and Kratos didnt intend to kill her. So Greed was still inside her.

The greedy Athena wants Kratos to give „Hope to her to remove the Chaos and return order to the world.(Just like Zeus and the other gods did after the Great War.) But Kratos dreaded what would happen if the fate of the mortals is given to a god another time. That is why he stabs himself! With that act, Kratos released Hope to the world and the mortals can decide for themselves what to do with it.

And Kratos?
Honestly guys, I have no clue. It is POSSIBLE that he is still alive. But Asmussen already stated that God of War III would be the end for Kratos. So even if there is a God of War IV, it is VERY unlikely that Kratos ist he protagonist.
Simply let him rest, my fellow Spartans. The Ghost of Sparta has earned his rest. R.I.P Kratos.