The setting is early middle ages, and the person being killed is not an enemy but a friend facing an inevitable long, slow death - the slit throat is intended as a mercy kill (incidentally, is there another more reliable/quicker/less painful/generally better way of ending someone's life with a sharp blade? Perhaps a severed cervical spine/brain stem? And if so, what's the best way to accomplish that?). From what I can gather, the best way of doing it is to stand behind and cut hard and deep enough to sever both carotid arteries, which aren't as close to the surface as the jugular but bleed out much faster. I'm not a doctor or an assassin, though, so I may be wrong. Also, how much force/how sharp of a blade does it actually require to cut really deep like that? How fast does death come? Is there another more reliable/quicker/less painful/generally better way of ending someone's life with a sharp blade (perhaps a severed cervical spine/brain stem)? And if so, what's the best way to accomplish that?
Search terms include "severed carotid arteries," "how to slit a throat," "best way to slit a throat," "slit throat forensics." I also found the youtube video of Clint Malarchuk getting his throat cut by a hockey skate, which is not for the faint of heart (although he survived, thank goodness).