The legitimacy of the Norman invasion and conquest of England by William I is usually discussed in terms of what went before - the promise of kingship to William by Edward the Confessor, and the relative claims of Harold Godwinson.
But a better understanding can be got from what happened afterwards - the reign of William the Conqueror and his descendants - and what that tells us about whether they were real monarchs, or merely glorified gangsters, tyrants and robbers.
The proper question is whether the Normans ruled as English kings, or whether they simply regarded England as a resource for their own exploitation.
The evidence of the events after the Battle of Hastings is perfectly clear - for many generations, the Normans did not rule as monarchs - as Fathers to the nation - but as thugs, tyrants and thieves whose actions showed that they despised the English and used the country as a playground.
1. The Harrying of the North in 1069-70. After putting down resistance from Anglo Saxon nobles - there was the attempt (within the constraints of time, technology and resources) to destroy Life in the North of England - men, women and children; animals; crops, forests, plant life.
Is it the act of a King deliberately to lay waste to a large part of the Kingdom?
2. Over the next generations, farmers were driven off vast tracts of productive agricultural land (to die of starvation); and these lands were made into wild forests for the exclusive recreation of the Norman nobles (their prey defended by arbitrarily conjured-up and draconian Forest Laws - e.g. amputation of hands for disturbing a deer).
The forestation of England - at the cost of the wealth and strength and well-being of the people - shows that that the Normans regarded the country as a playground, not a nation.
After the Conquest, England was for many generations a reduced, impoverished, smaller and feebler nation.
3. The endless, petty and pointless - yet murderous and destructive - squabbles and civil wars between the Normans (twenty years of fighting between Stephen and his cousin Matilda - only for Matilda's son's to inherit the throne...) and their descendants (right down to the Wars of the Roses) - show that the invaders regarded England as a mere larder, to be grabbed - if possible - for their own personal consumption.
We ought to judge monarch's primarily by their motivations - not by their attainments (which depend on multiple factors beyond control). Thus, a monarch can only be good when he or she has the country's good at heart, and rules on behalf of God.
Several or many of the Angle and Saxon Kings were good, by that criterion. But after 1066 there was a period of hundreds of years until we could be sure that this was the case again - it was probably not until Elizabeth the First that I would feel confident that there was a real monarch again on the throne of England.
The evidence is strong and it is objective that the Norman Kings and their descendants were - nearly all of them, and for centuries - merely successful and effective louts and looters; not true Kings: not even trying to be true Kings, because that responsibility (and love) would limit their license to grab, grab, grab.