In Knives Out, what was Ransom's motivation for wanting to nullify Harlan's will in the last act? Why did he bother to meet Fran and to kill her to frame Marta?
Marta had already agreed1 to give Ransom a share of the inheritance for keeping her secret. What did Ransom have to gain? If anything, voiding Harlan's will would have given Ransom a smaller share since the inheritance would have been split with the rest of the family.
By the time of the meeting with Fran, Ransom already knew that Marta had not actually done anything wrong and that the toxicology report would not indicate a morphine overdose. Therefore Ransom would have known that Fran had no actual leverage. If the toxicology report came to light, so what? The toxicology report would not have shown anything unusual and therefore would not have garnered much attention. Even if Marta became aware of its findings (and didn't dismiss the unexpected results as laboratory error), Ransom could have claimed that he managed to have the results doctored. Regardless, Ransom kept his end of the deal, so Marta would have had no reason to deny Ransom the share she promised him.
So it seems to me that Ransom could have just ignored Fran's blackmail note and kept his share of the inheritance.
1 Well, Marta never explicitly agreed on-screen, but it seems clear from Marta's and Ransom's continued interactions that an agreement was met.