E.T.-A monster..?

Being an amateur astronomer ,i have always been fascinated with the various theories concerning the existence of Extra -Terrestrial(E.T.) Life..But one thing which fascinates me is the human perception of E.T..Why, in every movie,every sci-fi book ,they are depicted as evil monster like beings..After all if "GOD" ,as per most of the philosophies (except Paganism,i suppose) has always had a heavenly abode..Then our approach to non-Earthlings must have been a positive one..

Strange.!!,is the way that human mind works and relates,isn`t it..?