Complex sentence: Walter, who Rose reveals is a vessel for her grandfather, attacks Chris, but Chris uses his phone's camera flash to free Walter from his hypnosis; he takes Rose's rifle and shoots her in the gut, then kills himself.
Split into simpler sentences: Walter, who Rose reveals is a vessel for her grandfather, attacks Chris, who uses his phone to take a picture of him. The flash frees Walter from his hypnosis; he takes Rose's rifle and shoots her in the gut, then kills himself.