Whenever a horrible crime is committed, Europeans try to understand why it happened, why he or she did it and then take measures trying to prevent it from occuring again. That's why the notion of responsability is so important over here. And because of "responsability", it is obvious , sometimes, the criminal cannot be held responsable, as events in his life can explain what he did and those events should have been faced by those around the criminal. Society can take care of them.
In the US, often, it seems that as long as a person doesn't do any suspecting thing, he must be left alone and when he does it, he's punished without asking question, in order to remove the problem, rather than face it. And besides, "thanks" to the ever presence of god, nobody wants to explain what he did other than by claiming he was sort of possessed by "evil".
Humans are very complex beings, and responsability is a social issue. It seems unfair but criminals are also human beings and anybody could become one. We must try to prevent this, not by hanging them, but by preventing it from the crime happening.

Raoul