While I hate it when the games lose their mojo with new installments, and it starts to look that they equate "sequel" with "bags of money" and nothing else, I really think I'll give God of War 3 a blind eye on this matter. Seriously, the first two games were absolutely brilliant and the third looks just shy of that, so it won't be hard for me to find excuses for Kratos and fill the story gaps with imagination myself. I mean, that's what games used to be about right? You have people who love the crap out of Rogue, and it's just a bunch of letters and signs dicking about on a black screen. If people can play that game and just forget about everything that's missing, I think I can enjoy GoW3 in the same way and have tons of fun while at it.
I prefer to believe that after Athena died (spoilers) in God of War 2, and Zeus just waved his dick in front of his face, Kratos kinda lost his shit and decides if Olympus will deny his vengeance on Zeus, than all of Olympus shall die (wait, he actually says so in the cutscene, so it's less "i prefer to believe" and more "he said so in the cutscene").
So it's justified that he doesn't really give a shit about any of the gods anymore. Or the world. He saved it so many times and yet he's still plagued with his past, so he decides, fuck it, it's going down. He's been tricked by Ares, Persephone, Charon, Icarus, hell, almost every figure from Greek mythology has just plain pissed him off.
So yeah, being a fan will help you justify anything, and it's hard not to be a fan of the God of War trilogy, when it's so spectacular.
But I am kinda annoyed with how unconvincing Hades looks. I mean, it's the place of the dead, once you die, you're not supposed to get out. And yet he gets out for the fourth time.