I think part of the issue, as with so much in the world these days, is the idea that there's a simple 'line in the sand' point at which these things kick in, and it's appropriate for everyone. There are ten year olds out there who know exactly what they're doing, and ten year olds out there who would be led astray in moments; the proportions change, but that's the same situation at 8 years old, at 12, 14, 16, even at 18 there are a few who haven't got there yet - that's obviously discounting the people with mental states and conditions which mean they'll likely never fully understand, but the system has a mechanism for determining if they're competent after the fact.
At various stages you can understand that things are wrong without necessary fully grasping the depth or subtleties of some of it, whilst you can have no idea that something else is wrong, and the specific combinations are a result of your development and upbringing - you can't presume all children will grasp one thing before another.
That's coupled with the US tendency to turn some of their police forces into paramilitary organisations in this specific case which makes it distasteful.
As to the James Bulger case, I find in some ways that's an interesting example so far as this discussion is concerned: Mrs O. watched a documentary on it some years ago (I apologise, I have no idea that channel or the name) and it included excerpts from the interviews with the two perpetrators. Quite apart from how distraught the whole thing made me, as the parent of (at the time) two children with another on the way, it was apparent to me that one of the two was absolutely horrified at what had happened, and the other was more concerned about trying to make sure that blame fell on the other: both of those reactions are a clear understanding that what was done was wrong.
O.