Gods tricked him into killing his family for power, gods use him by promising things they won't do to make the pain go away. The spartan is an idiot and falls for it like 4 times. He's pissed off because he's dumb, and proceeds to use/kill everything for vengeance.
COMPLICATED.
True. Well, except for the maidens I guess, but they still suck.
Let's face it, every god in that story with the (notable) exceptions of Artemis (who wasn't there for some reason), Hephaestus, and Hestia (I don't even remember if she was there) are dicks. Athena just managed to hide it till the end.
It's not a terrible series by any means; It has a very high quality to it. It's that everything ever is now attached to that series like it did everything for the action/adventure genre first, and everything has become a GoW 'clone' despite it not really being all that original in itself.
So basically, Hype backlash.
Honestly you've basically broken down every action game that has you actively fight your foes ever.
EDIT: Granted, GoW3 has a massive over-indulgence in DOSE CHAINS