In the later Middle Ages, this elision of law and morality was certainly one which kings, as lawgivers, attempted to make. But it was harder to convince people, because there were so many different kinds of law - in England for example, there was common law, customary laws, ecclesiastical law, forest law, maritime law, the laws of war, and so on. Which set of legal rules applied depended on place, context, and who was involved. Faced with such a diverse set of laws to choose from, in many ways it was far more obvious that law was a changing and historically contingent thing, far removed from what was supposed to be the more constant nature of morality. Lawgivers couldn't pretend to exercise the same moral hegemony and control.
Postscript: interestingly, Michael Adebowale has been charged with murder rather than terrorism offences after all (30th May). In terms of rejecting the horrific violence he carried out, this strikes me as a far more effective response than acknowledging that he has some kind of political platform by categorising him as a terrorist.