Medieval soldiers pretty commonly killed opponents who were wounded or too busy fleeing to fight back effectively, but this was more of a "finish the job" sort of thing than a mercy. Although the medieval soldier would typically hope to capture individuals of value in order to hold them for ransom, they had no qualms about killing outright and the skeletons of soldiers from medieval battlefields often show multiple (frequently gratuitous) wounds. Towton 25 here has clearly had his face smashed open by a poleaxe or something and also had a pretty serious cut on the back of his skull (which are the most fatal but definitely not the only wounds he received). It was probably fairly obvious that 25 was deceased or about to be when he received either of these, but his victorious opponents continued to hack at him, venting their rage after the bitterness of the civil war in England and the hard fighting of the Battle of Towton. All classes of medieval soldiers killed incapacitated or fleeing opponents eagerly. It was not restricted to any particular group, although footsoldiers would be more likely to carry out such a task just because they're on the ground standing next to wounded enemies already (while a mounted soldier would have to get off his horse to knife a guy on the ground).