It is hard to even imagine what it is to be decapitated or to see a decapitated body because to see that would mean to consider abandoning humanity. This is not a positive thing, but may not be as negative as you'd think in terms of dream interpretation.
A dream in which you witness a decapitation means that you are worried about something that you have done recently and you feel like you have a bad punishment coming to you soon. Perhaps you are feeling guilty over something you did that was morally wrong, or it is possible that you have broken an actual law, the latter is more common when this type of dream is the subject because you are afraid of punishment or retribution coming from a specific source. You feel like you are a little worried about what could potentially happen to you. This is one of the best things because you can at least feel a little bit less worried about the future. At least you don't have to actually worry about being decapitated in real life, all you need to worry about is getting some slap on the wrist or some dealings with the police. Hopefully whatever you did, legally or morally, was a simple small crime and you are over-blowing it out of proportion. This is the best case scenario.
Dreaming about a decapitated body means you are mortified about what might happen to you one day. You are worried that you could be brutally murdered. This is not to say that this view is founded on anything reasonable, you might just be afraid in general right now of something that might be happening to you. If you have a dream where this happens it means that you need to calm down a little, you have no reason to worry.