When you get a nightmare could the message in it be right? Some people have the ability to have for-spelling dreams. Imagine that someones'face changes into a devil and making a nightmare of it, could this be a dream revealing to the dreamer some information about other peoples state. 
Or can only demons'revealing dreams which are calm contain true messages? 

So this question concerns people who are usually gifted with visions etc.
Maybe, maybe not. 

Negative dreams indicate evil or negative thoughts inside of you.

Interpreting dreams is mentioned as a Wrong means of livelihood for monks (DN 2 and elsewhere).

If you want to have no negative dreams practice cultivating, developing, and pursuing loving-kindness towards all beings (metta).

It is mentioned as one of the 11 advantages of developing loving-kindness (metta):


  "Monks, eleven advantages are to be expected from the release
  (deliverance) of heart by familiarizing oneself with thoughts of
  loving-kindness (metta), by the cultivation of loving-kindness, by
  constantly increasing these thoughts, by regarding loving-kindness as
  a vehicle (of expression), and also as something to be treasured, by
  living in conformity with these thoughts, by putting these ideas into
  practice, and by establishing them. What are the eleven?
  
  
  He sleeps in comfort. 
  He awakes in comfort. 
  He sees no evil dreams. 
  He is dear to human beings. 
  He is dear to non-human
  beings. 
  Devas protect him. 
  Fire, poison, and weapons cannot
  touch him. 
  His mind can concentrate quickly. 
  His countenance is
  serene. 
  He dies without being confused in mind. 
  If he fails to
  attain arahantship here and now, he will be
  reborn in the brahma-realm. 
  
  
  (AN 11.15)


Therefore try to concentrate and pervade your entire body and mind with loving-kindness on a daily basis to gain these 11 advantages.