I agreed to a fault with your take on the series... UP until the end.
I was elated with the way in which the ending was handled, and offer a differing view.
I walked away feeling we finally saw Kratos' growth reach it's pinacle in GOW 3. He wasn't so much being pointlessly brutal, as his point wasn't realized yet, not really, not by him and possibly not by the player.
Like most people, Kratos was acting out through the majority of his time with godhood, lashing out more pointedly, through the entire series. In GOW1 he killed his family by accident in the original story and he had never been able to forgive himself for this. So he focused his rage on Ares, 'blame-casting' or 'projecting' his self-loathing. Killing Ares in the end did nothing but move him to a different place, different faces. He was made a god, but still all of GOW2 was just his inner demons, his self-hate continuing to eat him alive as he lashed out, finding new places to set blame for his pain and hate. And again, as in 1, he was not seeing where his blame was truly set. GOW3 finally brings this to fruition as he is forced to feel as a father again and lose a child again. His bloodlust brings the destruction of the entire planet and his rage still fuels him, leaving him no place to look, but where he should have looked all along, but as with most people, even the mighty Kratos refused to set the blame for his hate where it belonged and learn to let it go. His anger, his blood spattered rage at the entire world had to be thrown everywhere, he had to destroy and demolish until there was nothing left to destroy to see that the thing he'd really hated all along was himself. He'd allowed himself to become a monster, and it cost him the lives and the love of his wife and child.
The final act, his enraged pummeling of Zeus was the deed most necessary for his character as it was brutality called for specifically by the player (The need for the FPS shift) to let it go, to beat and beat and beat on Zeus until it was simply the tiring act it had always been. It WAS player control at it's finest, an act of letting go because we realized we were doing something pointless, acting on a rage against no target, just a red blur, hate for it's own sake.
And then, in the darkness he'd created, he finally learned to let it go. A fitting final narrative.
This is a behaviour we all see in a friend or perform ourselves. Be it the unresolved issue with the parent-figure resulting in directionless aggression all of someone's adult life ruining every romance and friendship, or the guilt over relationship trauma in one's youth resulting in an unending string of bad relationships and 'clingy' emotional uncertainty.
Personally, the worst part of the entire series to me? Seeing the blood trail in the end, implying Kratos was somehow not dead, and possibly leaving the franchise open for another sequel... dear gods I hope not.