With the Norman Conquest and the Witan abolished the monarch had greater authority in name his successor. This authority later evolved into a more concise hereditary system.
William I “the Conqueror” divided his lands upon his death in 1087. This was a common practice in this age. This practice was continued in many German states leading to many problems…but I digress. William gave Normandy to his eldest son Robert, and the Crown of England went to his next surviving son, William Rufus. The third surviving son, Henry, received 5,000 pounds in silver and there is a legend that says that the old king, William I, had declared to Henry: “You in your own time will have all the dominions I have acquired and be greater than both your brothers in wealth and power.”
William Rufus was therefore the new and lawful king and is counted as William II and ruled until his death in a hunting accident in 1100. By that time the “prophecy” about Henry had come true. Robert II of Normandy was on a crusade in the Holy Land in August of 1100 when William II died in a hunting accident in the New Forest. With Robert far a way Henry claimed the throne. Did Henry usurp the throne from his brother Robert?
There had been an agreement between Robert II and William II to become one anothers hier should either of them die without issue. This was made in 1087 upon the death of the conqueror. This brotherly love did not last long for in 1088 the two rebelled against one another. This rebellion was spurred on by the barons in both Normandy and England. Dividing the lands between the two brothers created a problem for the barons. The Barons owned land in both England and Normandy and having to serve two different rulers created many difficulties so they sided with Robert II to defeat William II and take the Crown of England from him. The major problem with this rebellion is that when it became time Robert II did not arive at the battle for Rochester Castle and the rebellion swiftly ended.
From that time on who the legal successor to William II should be was never settled. The Crown was up for grabs until William could name a successor. He never got the chance. When he was killed in a hunting accident and with Robert too far away from England to claim the throne, Henry siezed the opportunity and was crowned King of England. In 1106 Henry I of England took Normandy from his brother thus unifying England and Normandy once again.
From a legal standpoint it seems that Henry can be considered the legal hier since the 1088 rebellion dissolved any agreement between William II and Robert II and in the abscence of a named heir the crown went to the first person to grab it.