Is the testimony of the executioner who carries out the death penalty in court acceptable, and has a Prophetic حديث (saying) been narrated that prohibits accepting his testimony?

There is no impediment to accepting the testimony of the executioner or others who carry out Shar'i rulings, if he is upright and executes only what results from a just verdict. The mere execution of rulings does not invalidate the testimony, as the Companions used to carry out rulings of execution and flogging. The scholars stipulated for the acceptance of testimony: Islam, sanity, puberty, freedom, alertness, uprightness, and absence of accusation. No hadith (definition) or narration has been reported that prevents the acceptance of an executioner's testimony.