Mother Teresa tells a story of walking past an open drain and catching a glimpse of something moving in it. She investigated and found a dying man whom she took back to a home where he could die in love and peace. 'I live like an animal in the streets,' the man told her. 'Now I will die like an angel'.
In his essay on the uncanny, Das Unheimliche, Freud said that the uncanny is the only feeling which is more powerfully experienced in art than in life. If the horror genre required any justification, I should think this alone would serve as its credentials.