Family Feuds
Harlan does not have good relationships with any of his family; in fact, it's not surprising that he wrote them largely out of his will in favor of his much nicer nurse, Marta. The fact that his family are still surprised by this course of action speaks volumes about them, and the kind of people they are. They feel that despite their behavior they are still entitled to their father's estate, which sort of sums up why he disinherited them in the first place.
Harlan doesn't like his son in law because he's a cheat, and is having an affair whilst married to Harlan's daughter ,Linda. He threatens to expose his cheating, and he does not like his daughter-in-law much either, because she has been stealing from him. He has cut off his son, Walt, and fired him from the family company. His relationship with his grandchildren is no better; he and his grandson are fighting.
Where there's a will, there's a relative
Harlan's children are more interested in him now he is dead than they ever were when he was living and each goes to great lengths to try to get their hands on the money in his estate. Ransom, his grandson, thinks outside the box, befriending Marta so that he can split the inheritance with her down the middle. The older Thrombeys put a lot of pressure on Marta to renounce the inheritance. Walt goes a step further and threatens to report Marta's mother to authorities for being an undocumented immigrant.
Evidence Sometimes Lies
Not all evidence pointing to guilt turns out to be evidence pointing to guilt. Sometimes, a piece of circumstantial evidence is just circumstances that conspire to make a person look like they have done something illegal when they have not. For example, Marta assumes she has killed Harlan because she knows she lost concentration when preparing his medication, and that he died after this. She concludes that his impending death is her fault, and assumes the identity of murderer, when in fact, she didn't make a mistake at all. She gave him the correct medication and played no role in his death.
Benoit sees blood on Marta's shoes and believes it suggests she killed Harlan, but although she was there, and his blood splashed onto her footwear, she was standing next to him during his suicide, and did not participate in his death. Similarly, when Harlan's mother sees Ransom climbing down from Harlan's room, she assumes he is escaping the scene of the crime after committing her son's murder.
