It could be their social standing. If they were well - respected in the community, like a church leader or a successful businessperson, people would not suspect them easily. They were probably seen as having too much to lose to engage in such a heinous act. Also, if they had a family and a good reputation, others would find it hard to believe they could be a murderer. Their daily life might have been so ordinary and positive - looking on the surface that no one would think they were capable of taking someone's life.
Well, without more specific details about this 'The Unlikely Murderer Based on True Story', it's hard to say exactly who the murderer was. It could be someone who seemed completely harmless or had no obvious motives at first glance.
It is indeed a true story. The creators did extensive research and documentation to present the events as accurately as possible. However, some creative liberties might have been taken for dramatic effect, but the core of the story is based on reality.
I don't really know. Maybe it started with a real story and then got embellished for the sake of the narrative. To know for sure, you might need to look into the background and the author's sources.