In October 1479, Gilbert Heg of Lambeth, Surrey, assaulted a constable, presumably in Lambeth. He struck him with a dagger known as a wynyard, wounding him in the chest. Gilbert claimed that he did this in self-defence.
Five days after their encounter the constable died – and so Gilbert was on the hook for homicide. A self-defence killing was still a homicide, so he could be arrested and stand trial. Sometimes juries acquitted in obvious self-defence cases, but the proper procedure was that a person who killed in defence of their own life would ask for (and normally receive) a pardon. But rather than chancing an acquittal or pardon for what he said was a self-defence killing, Gilbert fled north.
On 4 November 1479, he asked for, and received, asylum at Durham cathedral.
Sanctuary Seekers in England, 1394-1557 