Knives Out is in many ways a traditional whodunnit; it is the board game Clue with a side of C.S.I. and a little Raymond Chandler thrown in for good measure. As in many murder mysteries, the character who seems to be innocent is guilty as sin, and the perceived murderer has not committed the crime at all - or has she? At the end of the film we are still not really sure of Marta's innocence, because whilst she did not poison Harlan after all, and does not have blood on her hands, she certainly has some on her shoes, suggesting that she was most definitely involved in the throat-slashing fury that helped Harlan meet a rather unpleasant end.
Ironically, Harlan need not have died at all. He slashes his own throat because, like Marta, he believes that she has administered the wrong medication, and knows he is about to die. He does not want her to be blamed, and so he makes his death appear to be a grisly murder.
There is a saying that where there is a will, there's a relative. Harlan's relatives waste no time in trying to convince Marta to renounce her inheritance, and they also realize quickly that it behoves them to believe in her guilt. If she murdered their father she would become ineligible for the inheritance. Walt tries to blackmail her into renouncing; Ransom tries to befriend her into giving him half by offering her protection - which of course turns out to be anything but, as he tries to frame her for the murder, and frame her for Fran's murder as well. The knives that are out for much of the movie turn out to be the knives that Ransom is using to stab Marta in the back.
At the end of the film, the rightful cause of death is documented, and Ransom is revealed to be Fran's killer. In this way the film is also a traditional whodunnit murder mystery, when the killer is brought to justice and the innocent are proved to be so.
