Brereton was a friend of Henry’s and a favorite. He was a groom of the Privy Chamber. He was also close to the Duke of Richmond. He had vast holdings in Cheshire and Norther Wales. He had a great deal of power there. He promised to be an obstacle in settling the Welsh border. He was on the wrong side of Cromwell. Brereton blamed the death of one of his men on a man named John ap Griffith Eyton. He sent Eyton to London to be tried, but the man was aquitted. Brereton had him rearrested, possibly with Anne’s help, and he was sent to Wales where he faced a rigged trial. Eyton was found guilty and summarily hung. This was done in spite of Cromwell’s efforts to save Eyton. And that is only one incident. Brereton was an irritating man to Cromwell.
This is a very brief summary of William. He was in his 40’s when he died. There is a big chance that Brereton would have run afoul of Cromwell and Henry over Wales eventually had he lived.