After Marta is revealed to be the sole inheritor of the fortune, Walt confronts her with the threat of having her mother deported, saying that if she gives the money back to the Thrombeys, they can use their resources to help her. Except... as Marta points out, "their" resources are now her resources, and she can get her mother a good lawyer herself, thank you very much.
What makes this moment especially awesome is that prior to Marta's realization, the camera work did a very good job of depicting Walt as imposing and on the verge of threatening. Despite having been intimidated at the start of the conversation, Marta was able to rally and face Walt down regardless.
Faced with an OD'd Fran, who is apparently blackmailing her for murder, Marta has the chance to just walk away and let her die. But after a moment of uncertainty, Marta calls 911 and begins giving Fran CPR. This later ensures that Ransom can be baited into admitting killing Fran. As Blanc puts it, Marta accidentally ensured that she won the game by being what she always was: a good person.
Tiny one for Fran; if you listen to her dialogue, she wasn't blackmailing Ransom for money. Benoit deduces that she was angry about finding out he tried to murder Harlan and was framing Marta for it. While her confronting him in an abandoned laundromat with no one knowing where she was proved her Too Dumb to Live, she holds on long enough to tell Marta there's a backup copy of the report and who the killer is. Sadly, because of her garbled pronunciation and Marta's guilt, Marta didn't understand the last part until much later.
The fact that she knew to have a backup copy of the report and hide it in a place only she and Marta knew the significance of shows that she was at least smart enough to think ahead in case it fell into the wrong hands.
After everything that the Thrombeys have put poor Marta through, Blanc lays into them with a very well-deserved "The Reason You Suck" Speech.
Marta: You've always been good to me. And what I'm about to say isn't going to be easy, and you're going to be upset. But I thought after what you've gone through the last few days, that you deserved to hear it from me. I... Blanc: Excuse me! You have not been good to her. You have all treated her like shit, to steal back a fortune that you lost, and she deserves. You're a pack of vultures at the feast! Knives out and beaks bloody! Well, you're not getting bailed out, not this time. Miss Cabrera has decided, definitively, not to renounce the inheritance. Walt: What? Marta: What? Blanc: Furthermore, it will be my professional recommendation to the local authorities that the manner of death, in the case of Harlan Thrombey, is ruled as suicide, and the case is closed.
Blanc's summation of Ransom's whole plan. Especially when Ransom starts out smug about Benoit spinning these conjectures, only to go Oh, Crap! as the detective accurately lays out what happened starting the night of Harlan's birthday.
Daniel Craig's performance is magnificent, selling how Blanc has been onto this for so long and playing a fantastic game to outwit the supposedly master player Ransom. That even Marta had no idea Blanc was on her side this whole time is amazing.
Marta's gambit at the end. The set-up: She vomits whenever she lies. Throughout the film she vomits into cups and pots out of view whenever she lies. After it is revealed that Ransom was the one who poisoned Fran, she gets a call from the hospital, and tells the police and Ransom that Fran is okay and she'll be able to confess. Ransom angrily says "Yeah, I killed Fran, but I guess I didn't..." and states that he won't be put in prison because all they have on him is attempted murder. Marta then proceeds to vomit chunkily all over Ransom's face, revealing that she lied — Fran actually did die, making Ransom a murderer. Way to make use of a crappy condition!
As Ransom registers this in stunned silence, Wagner just as silently (but far more triumphantly) shows him that he was recording the whole thing and has his confession on his phone.
After being found out, Ransom tries to kill Marta, grabbing one of Harlan's display knives. The music gets intense and everything goes into slow motion as the police and Blanc try to stop him from stabbing her, only for him to plunge the knife into her chest...at which point it is revealed that the knife is a prop knife (set up earlier by Harlan stating that Ransom doesn't know the difference between a prop knife and a real one). Realizing this, Ransom only manages to utter "...shit..." before being arrested for the aforementioned murder and attempted murder!
The final scene of the film. The family gets one last look at the estate as Marta literally looks down at them from the balcony, sipping her coffee. The cherry on top is how utterly calm and collected Marta is throughout the whole thing. No gloating or obscene gestures, even though, after everything the family put her through, both would've been very justified. No, Marta remains civil and polite, even as she watches those parasitic backstabbers leave her mansion. What's even better is the symbolic implications of the last shot, as pictured on this page. The mug she is using is the same one that was taken to Harlan's room, but her hand is blocking out two of the three phrases painted on it:
Harlan revealing in a secret letter to Linda about Richard's affair.
After the entire mystery is cleared up, Benoit Blanc reveals he knew the whole time Marta knew more than she was saying due to spotting the blood on her shoe, but rather than confront her about it let her string him along just because of a feeling that whatever she was hiding, it wasn't as bad as murder. After Marta had earlier mocked him having to be handed the key piece of evidence by the culprit, he gets to show just how great a detective he is.
Faced with the prospect of dying in minutes of an apparent morphine overdose if Marta doesn't find the antidote, Harlan notes down how being poisoned by switching medication would make a good plot for a future book.
Once it becomes clear that Marta doesn't have the antidote, Harlan concocts a plan, in seconds, that not only protects Marta and her family, but ensures that his death doesn't invoke the Slayer rule that would deny her from inheriting his wealth.
Take it with a grain of salt, but considering what a complete asshole prick Ransom is, for him to look up at Marta as he's being carted off to jail and give her a respectful nod for outplaying him at his own game, and she acknowledges it in return, is strangely pretty awesome.
Marta hiding the evidence of her following Harlan's instructions during Blanc's initial investigation. Security tape footage of her? She "accidentally" ejects the tape and breaks the VCR, pockets it "for the detectives," and subtly wipes it with a fridge magnet as she hands it over to them. Open gate in a muddy path with her footprints clearly displayed? She heads up the path before any of the detectives get there to make it look like she opened the gate just then, and when they tell her to not use the path to preserve any footprints, she pretends she can't hear them and walks back down the path to obscure her own prints. She finds the wood that fell off the trellis? She tosses it into the distance to make it look like she's playing fetch with one of the dogs.
Great-Nana gets an offscreen one when she finally tells Blanc what she saw on the night of the murder, thus ensuring that Marta gets her inheritance fair and square while the culprit is taken to jail. She does this because Blanc, unlike the rest of her selfish relatives, took the time to show her sympathy and kindness after her son's death. Given that she laughs during "The Reason You Suck" Speech described above, it's heavily implied that Great-Nana was practicing Obfuscating Stupidity and knew her family members were a group of conniving, money-hungry jerks who treated her like dirt...and now she's just gotten the ultimate revenge on them. The Dog Bites Back indeed!
