Correct me if I'm wrong, but the story indicates that he beat the elderly woman, he didn't just hit her. Not to death, but he beat her just the same. If you assault someone severely, even if you weren't trying to "finish the job", the law holds you responsible if they die, and rightly so.
Even taking that all into account, 10 years old should be old enough to know not to violently assault a frail elderly person. I agree that intent should be factored into any punishment handed down, it would be worse if he was actually trying to kill her. But it's still a heinous crime that a kid that age should know is wrong.
As for the media violence angle, I'm definitely sympathetic to impressionable young kids that grow up on violent video games and action movies. It's definitely a mixed message we send to the youngsters and we as a society should not do that. Still, it's no excuse for violent crimes. We can't have kids running around bludgeoning the elderly without repurcussions because their parents let them play COD.
Edit: YMMV may vary on what a "beating" is. He used a weapon to restrain her and hit her multiple times; that's a beating in my book. Some may disagree if it was only a couple of strikes. I stand by my overall point in either case.