First rule for writing in horror movies (and sometimes thrillers); do the unexpected and you get surprisingly scary results. After the genre's initial release, audiences eventually got smarter and started to gain an idea about what to expect in horrors, specifically things like "don't go in the basement!" or "whatever you do don't split up" and many more courtesy of Scream and Scooby Doo. Once upon a time, a writer somewhere in a creepy decrepit basement must have figured out that the best way to scare people was by turning the ordinary, harmless and lovable things in life and make them vicious, vile and pure evil at its core.
How on earth does that work you ask? Well as a writer, when you've almost exhausted your material, you need to expand your territory to new heights in order to keep things interesting. Remember the first rule I talked about? The genre had to evolve in order to keep up with the times, so after many, many, many cheesy low budget horror films, there finally came a new concept. Nobody would expect children, their dolls, nursery rhymes or innocence in general to freak you out; which is exactly why they became the perfect and surprising scare. For example, twins are not usually scary, but put them in the right place at the right time, add a little blood on the walls and make them speak in a creepy unison and you've already left the theatre. Horror keeps on continuing to be a nerve wrecking (and enjoyable) experience because it never fails to create new fears and exploit them - it just so happens that cute and cuddly is the new scary medium.
See that stone angel above. It looked perfectly normal when you weren't looking, but now... you blinked and it is terrifying. You might never see statues in the same way again. Now you see, even the little things that shouldn't disturb you like a lawn ornament now keep you on the edge of your seat constantly looking at the corner of your eye.
Although, this sugar and spice premise only really works when there is a tense atmosphere to heighten the mood by making you feel like your imagination has gone wild with fear. Music has always been a great way of doing this for anything in general and it still applies here.
At the time of the release of 'Maleficent', the trailer was accompanied with the song Once Upon A Dream by Lana Del Rey which is a eerie rendition of a classical piece used in a Disney film (Beauty & the Beast I think or Sleeping Beauty, what do I know?). If you watch and listen to the trailer and/or song, you'll understand how much of an impact the musical score has on the atmosphere of the movie and the personality behind Angelina Jolie's character.
What was originally a beautiful song, has now been re-purposed to fit a movie from the darker perspective of the antagonist in a sinister twist compared to the original child's tale. The same also applies to the recently released trailer for 'The Avengers: Age of Ultron' which if you didn't notice, the song and dialogue was referring to Disney's Pinocchio. This childish and innocent element is unnatural and thus gives a desired creepy effect.