The jury (and perhaps the audience) flips between believing Sandra and being more skeptical as evidence is introduced that the head injury that killed Samuel occurred before he hit the ground, that Sandra and Samuel had a physical fight the night before he died, that he was deliberately tormenting her with his music on the day of his death, that hed accused her of plagiarizing him, that she had been unfaithful in the past, that he had possibly attempted suicide on a previous occasion, and that a subplot in one of her books involved a woman contemplating killing her husband.

The main possibilities the movie poses are that:

1. He fell. Sandra says she thinks this is what happened, but the height of the window ledge, the trajectory of the fall, and the lack of blood splatter on the shed roof suggests that this might not have been the case. The lawyer Sandra is friendly with thinks the jury wouldnt believe this anyway (the lack of blood on the roof, though, could have been due to the snow on that section melting, we later see).

2. She killed him. Shes annoyed with him, hes accused her of a bunch of things, shes unhappy in France, they had a fight, maybe it was an accident in the heat of the moment.

3. He killed himself and tried to set Sandra up for his murder. In the recording where the two argue, Samuel very much provokes that fight. She tries to talk him down repeatedly while he brings up things from their relationship which occurred years previously but are the clear flags for what her motive would be. She tells him she loves him. But he deliberately pushes her until she reacts physically. She says she threw a glass at the wall and slapped his face, and he grabbed her wrist (leaving bruises). She then explains further sounds of violence and bruising on his body were him hitting himself. We dont know if this is true, but if it is, a reasonable assumption could be that hes trying to set her up.

4. He killed himself, simply because he was unhappy and with no additional motive. We know he had been taking antidepressants in the past, had taken an aspirin overdose once before (verified by Daniel who gives poor old Snoop aspirin again before he testifies, to recreate the way the dog was behaving after the time Samuel took the overdose and Snoop had eaten some of his vomit). And Samuel had a very strange conversation with Daniel in the car talking about how Snoop wont be around forever. Or did he

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An Anatomy of a Fall Theory That Changes the Meaning of the Ending - Den of Geek