Legally, this is a cut and dry case. The man committed a crime, on American servers and therefore should be extradited, that's just how the system works.
If some French person had organised a bomb attack on the tower of London, even without stepping foot on UK soil then the result would be the same, extradition.
Whether he is mentally competent or committed the act with malicious intent is for the courts to decide, as is the length and harshness of his sentence.
I know American law may seem uneccessarily harsh but does anyone doubt he will get a fair trial? Does anyone think that American courts will be unfairly biased against the poor bloke?
No, he will be treated just as fairly as he would be in Britain.
He should be extradited, given a fair trial and if the jury returns a guilty verdict then he should be punished proportionately to his crime.
That's the rule of law kids, it may not be nice, it may seem harsh but it is there, it is fair and it holds us together.
Now, the lopsidedness of the extradition treaty between the UK and US should be sorted out first, but that's a different story...