Let's get a few things across, I don't believe the game is 'very' gorey, I also disagree with the rating the game received. I don't believe this game is too adult, even for small kids.
That being said, let's get a few points across.
It IS bloody.
There IS some gore.
There IS some language.
There IS some slight biblical elements. (emphasis on slight)
It CAN be disturbing.
But to that point. Is the gore graphic? No, it's there, but it's not realistic or even very detailed.
Is the language intense? Not really, mostly damn and hell.
Is the game sacrilegious? The game clearly has some inspiration, but it doesn't mock or mimic anything from the bible.
Is it frightening? Not in my opinion.
Diablo 2 is not a game I'd of wanted a small kid to play, the 2D graphics managed to pull off some fairly realistic and disturbing looking gore and creatures, but 3D has not kept the horror nature of the diablo series very well.
Diablo 3 is all about the action. My kids have never even pointed out any of the few true disturbing parts of the game, in some cases it went over their heads. (Like a guy asking you to meet his wife after saving him, only to find out he's delusional and his wife is a skeleton).
It's a game where the kids are less interested in the environment and more interested in how to play, how to beat the bad guys efficiently, what skills to use, how to beat the bosses.
I considered keeping Diablo away from them for the reason that it might be too adult, but I have never seen my kids get along better than when they play Diablo. My 11 and 9 year old have no clue how to work together, but when they play Diablo 3 that all changes. They'll help each other figure out the menu, give each other advice on gear and actually make strategies how they should proceed in the game.
They make sure to share gear that the other could use, they WANT to play with each other and not alone.
It's a game that encourages and rewards teamwork very well.
I think most people would think they're a bit young for the game, but the way it has helped their relationship as siblings has made it our family's favorite game to play on game night.