Suffer-Not-Injustice Vimes
The last king of Ankh-Morpork was Lorenzo the Kind, and his reign was ended with an axe wielded by Suffer-Not-Injustice "Stoneface" Vimes, the then-commander of the City Militia (predecessor to the Ankh-Morpork City Watch) who was also, technically, defender of the crown. He then went on to rule the city for several months. Vimes was of a noble family, but his name was disgraced after the killing- after all, even if he did what everybody else wanted to do, he had still killed a king, and without trial.
Stoneface's case was re-evaluated by the Guild of Historians at Lord Vetinari's request. The name of Vimes has been re-established along with the coat of arms, much to the bewilderment (and annoyance) of his descendant Samuel Vimes. Commander Vimes is now forced to use a noble title and a coat-of-arms, but he insists that he be called "Commander". However despite the re-evaluation of history, many still see Stoneface as the murderer of the last king of Ankh, and so it may take time before he is exonerated.
Samuel Vimes seems to have also inherited Stoneface's nickname, and some of his officers call him "Old Stoneface" when they think he can't hear them.
Annotation
A reference to Oliver Cromwell (in Feet of Clay, who was known as "Old Ironsides". Old Stoneface was described as having "warts and all"' (a misquotation of Cromwell, who desired that he be painted "warts and everything" by Sir Peter Lely). After having King Charles I of England executed, Cromwell was himself later ritually beheaded. Although he died in 1658, in 1661, Oliver Cromwell's body was exhumed from Westminster Abbey, and was subjected to the ritual of a posthumous execution by beheading. Symbolically, this took place on 30 January; the same date that Charles I had been executed. His body was hanged in chains at Tyburn. Finally, his disinterred body was thrown into a pit, while his severed head was displayed on a pole outside Westminster Hall until 1685. The head was eventually obtained in 1960 by Cromwell's alma mater, Cambridge University's Sidney Sussex College, where it is buried in a secret location.