Seeing people getting decapitated seems a fairly routine and neutral thing in my dreams, last night someone I knew from uni accidently beheaded someone else with a hocky stick, so the beheaded person (who I don't know) came back and beheaded the uni person somehow... I had a dream a while back where people from uni were sitting in a circle in the common room swapping heads just to see what it was like having other bodies - I remember holding the head of a guy from there called Lewis, who I quite liked in some ways, but we had trouble getting his head back on in time, and it started to die.
So far, I haven't been the one decapitated, and it's always quite a calm thing, it certainly feels like any regular dream