With respect (because I do respect you) I can not take this with the serious bent you do. I do not believe in the supernatural.
If one receives such ominous and threatening texts, what does a reasonable person do? They contact the police or service provider, in order to find out who it is. In Canada it is a criminal offense to threaten anyone, this is food for the cops. From the narrative of the story you provided, the recipient plays the victim in a teen horror movie. They passively receive such texts and allow themselves to be victims.
Counterpoint. If demons are texting people why not angels? I have heard theists claim they have an angel watching out for them. How about “I know you are going shopping at 2:00 PM. Please be careful in the parking lot, there will be a crazy fast driver who may come close to you. Your friendly angel”.