According to most authorities he was the son and heir of Robert, Count of Eu,[a] (died before 1093), by his wife Beatrix de Falaise.
Although he made his peace with that King, together with William of Aldrie (his wife's nephew), Roger de Lacy and Robert de Mowbray, he conspired to murder William II and to replace him on the throne with Stephen of Aumale, the King's cousin.
In 1095 the rebels impounded four Norwegian trading ships and refused the King's demand to return the merchandise.
During January 1096 in Salisbury, William was formally accused of treason, challenged to trial by combat, and defeated by Geoffrey Baynard, former High Sheriff of Yorkshire, acting as the King's champion.
[3] William's son Henry inherited the countship of Eu and also became Lord of Hastings.