A one dimensional character does not mean that the character only has one trait. It means a character has one trait that is so overbearing that any other complexs about the character are overshadowed.
Kratos seeks vengeance because of his losses, the trickery, lies and deceit. He lost everything, you can see it in his voice when he talks to Atlas in Chains of Olympus. Atlas says, "You believe in the Olympians." Kratos replies, "Its all I have, Atlas." His voice breaks. Its telling and as someone who has played the games multiple times I believe I have better insight than the ones saying he is one dimensional.