The Evil Within (2017)

The mentally handicapped thirty year-old man Dennis lives with his older brother John. When John decorates his room with an antique mirror, Dennis has nightmares and finds evil in his reflection. His reflected image forces him to kill people including his beloved ice-cream seller Susan and Dennis becomes a serial-killer. Meanwhile John's girlfriend Lydia decides to move in together with John and Dennis ignoring the threat of her brother-in-law.