Am I alone

Am I alone in wondering how it is possible that the defendents in the murder trial of soldier, Lee Rigby, have been permitted to plead 'Not Guilty'?
That they are guilty is surely not in doubt so why put the family of the deceased through the trauma of a trial and the Nation, presumably, through the massive expense.
Is it because of the stupidity of British law or is it the greed of barristers trading on human grief by defending the indefensible?