I'm not sure that this is a safe assumption - the reverse could easily be assumed: that encounters with a being of light are fakeries organised by the darkness to "bind us" (with apologies to Tolkein).
1) Most NDE's follow the positive pattern.
2) At least some hellish NDE's seem to convert to the positive form part way through.
3) I don't know of a positive NDE that turned bad part way through.
Maybe someone can add some more concrete thoughts on this.