There are some movies that cause a visceral reaction, that lodge into your subconscious for days on end and leave a lasting scare. These movies also age well, and offer the same feeling of dread and helplessness years later. I last felt this way with the original remake Grudge of 2004, to the point of sleeplessness for days. Since then I became immune to horror as it's a steady diet of ours, but Sinister scarred again and opened old terrors, even after a rewatch years apart.
The movie disturbs on multiple levels, particularly in the thought that innocent kids can be turned into monsters of unimaginable cruelty, a sad parallel to the world today. In the case of Sinister it's particularly terrifying to imagine what ancient evils [and demons] lurk amongst us, silently moving from one victim family to another. Just how many crimes go unexplained and filed away into cold case status, with supernatural aspects that no one wishes to root cause or cure.
The found footage aspect of Sinister is heart chilling, the cold methodical way in which entire families are put to death. All the time you swing between pity and disgust at the character played by Ethan Hawke, an obsessed writer aware of the impending danger but unable to resist the temptation to exploit it.