One lesson could be the importance of being cautious when dealing with strangers. In the story, if the character was too trusting at the beginning, it shows that we should be more aware of potential risks.
Since I don't know the exact story, but if it's a common 'at the mercy of a stranger' type, perhaps there was an accident or a difficult situation where the main character had no choice but to rely on this stranger. Maybe the stranger offered help but with some ulterior motives. It could be that the main character slowly discovers the true nature of the stranger as the story unfolds.
Yes, 'Mercy Black' is based on a true story. It often takes real - life events and weaves them into a horror narrative to create a more chilling and believable experience for the audience.
I don't think it is. 'Have Mercy' sounds more like a fictional tale designed to engage the audience with an invented plot and characters rather than being rooted in reality.