Verden, Saxony, in the year 782. Charles, King of the Franks orders the execution of 4500 Saxon prisoners of war.

This was the same Charles, known to history as Charlemagne, who wished to convert his domains to the Christian religion, who was the close friend and protector of the Pope, who implemented a widespread education system based on the most enlightened traditions of the Ancient Greeks and Romans, who had invited so many of the greatest scholars, thinkers and writers to his court that future ages would come to refer to the period as 'the Carolingian Renaissance'.
Charlemagne was one of the many powerful leaders of a united people who, while being loved by friends, followers and family, were nevertheless capable of acts of the most ruthless brutality.
In post-Enlightenment, western civilisation no mitigating circumstances can be morally, or legally admitted for mass-murder, but our more secular age still seeks rational explanations for criminal acts. In the absence of conclusive evidence, these can fall within the province of speculative, historical fiction.
Charlemagne's was an age of extensive written sources, including a contemporary biographer of the king himself and a series of court annals, so any speculation would not be entirely without foundation.
Psychopathology doesn’t explain Verden. Charlemagne doesn’t seem
to have been a prey to any identifiable psychopathic condition. He was not in
the grip of an inherently iniquitous ideology. In fact, many of his Christian
advisers like Alcuin of York were men of peace who exhorted the king to be
moderate in his acts of governance. However, there were Christian advisers of
another kind, like the fearsome Archbishop Tilpin who rode into battle with the
king to do his share of the fighting. Which influence held sway at the time of
the massacre?
Charlemagne had subdued and formed the most extensive empire
since that of the Romans. His conquests included Aquitaine, Lombardy, and
Bavaria. With one notable exception he was used to winning. He seems to have
been haunted by a humiliating defeat at the Pass of Roncesvalles in the Pyrenees
when a large part of his army (and not a few loved ones, friends and notables of
his court) was annihilated.
In the spring of 782 Charlemagne held an assembly in Saxony attended by many
members of the Saxon leadership. On his return to Francia, he sent an army into
Saxony to repel an invasion of Serbs. This army found itself fighting not Serbs,
but Saxons. The Serbian invasion was part of a trap. The Frankish army, poorly
led in the absence of Charlemagne, was defeated. Did anger at the way history
repeated itself sweep away the king’s moderation?
These Saxon Wars had been going on for ten years. They were seen
by the older Franks as a continuation of the campaigns of Charlemagne’s father,
Pepin the Short, and, as such, a point of honour that they be concluded by
victorious conquest. Charlemagne must have known that the nobility were discontented with the Saxon situation, amongst other sources of discontent. Charlemagne’s official
biographer wrote that no struggle was ‘more prolonged, more bitter, or more
laborious’. Ten years after this defeat Charlemagne’s eldest son would lead a
rebellion of a disaffected faction in an attempt to overthrow his father. Was
the king under irresistible pressure from powerful members of his court who
thought it was time the matter was settled once and for all? The king put
himself at the head of another army and invaded Saxony.
The year after, Charlemagne suffered a double bereavement. Two
women were powerful influences on him at this time – an adored mother and a
gentle, loving wife. Within seven months of the latter’s death, he married
again. This new wife was anything but gentle and loving. In fact, she was
renowned for her cruelty. The speedy nature of this remarriage might suggest
Charles had come under the influence of this new wife before the death of his
Personal factors, political pressures, conviction, ideology,
cultural preconditions...all these can distort individual moral judgements.
Which of these impelled the massacre at Verden?