Assane's entire life really. His father committed suicide when he was just a kid (which turns out to have been a set-up; he was murdered by Léonard), he was left completely alone as his mother was in prison in Senegal, and throughout faces racial discrimination. In part 3 it is revealed this all led to him joining Keller's 'boxing club' (really a place adolescents without homes can live in without arousing the suspicions of social services). When Keller makes Assane fight Bruno in a battle of dignity, Assane starts to realise who Keller really is, but is unable to get out of the situation. This eventually culminates in Bruno being forced to kill a police officer by Keller after a robbery, with Assane in the back seat desperately trying to stop him but failing, leading to a horrific car crash. Assane puts the gun in Keller's hand, sending him to prison for 25 years, leading to the vengeance he seeks in part 3. Assane was a teenager at the time. Knowing this, it puts Assane's more selfish decisions into a sympathetic light, and highlights how he turned to the Lupin books for comfort and idealised the main character, leading to his signature replication of the novel events.
Assane's reason for his thievery, after finding out his mother is in prison for robbery:
The death of Babakar, Assane's father, seen in the first episode. Assane as a teenager is waiting to visit his father, who is in prison for his apparent robbery of Pelligrini's necklace. The shot showing him hanging from the bars of his cell hides nothing, and it is left to the viewer to consider Assane's reaction that his father, the last family he has left in France, is dead.
Fabienne's death. Oh, sweet Fabienne. Her refusal to reveal Assane's location directly led to her death, being killed at the hands of Léonard which he framed as a suicide. Assane arrives not long later, with J'accuse coming to find him, whimpering. The entire scene is heart-breaking; although she was a Tragic One-Shot Character, her friendship with Assane along with their quiet banter are some of the most heartwarming moments in the series. His expressions perfectly encompass the shocked grief and guilt he feels, holding J'accuse closer and crying silently into his fur.
Raoul's kidnapping. Just seeing the distress of a child being forcefully taken is enough to get the tears going, but seeing him tied to a chair in the abandoned manor makes it even worse. It's taken even further when Raoul is shut in the boot of a burning car by Léonard, which Assane sees explode. In Assane's eyes, his son has just died a horrific death right in front of him, and it's all his fault. Luckily, Raoul was freed just in time by Guédira, but this is not revealed until after we see the car explode, leading the audience to believe Raoul actually died.
For all the Black Comedy of Guédira interrupting Assane's funeral to demand that the casket be opened, the shots of Claire weeping after seeing Assane's body and literally leaning on Raoul (who appears to be experiencing a form of Heroic BSoD) for support really drives home how devastating it is to suddenly lose a loved one, even if that person has been estranged for a long time.
Assane's betrayal of Benjamin. More specifically, Benjamin's facial expression when he runs into the dead end in the maze is a highly upsetting mixture of horror, shock, and the dawning realization that his best friend has sold him out. As he's handcuffed and led out of the castle grounds by Belkacem and Guédira, he has visible tears in his eyes, and looks completely broken.
Claire eagerly waiting for Assane at the train station, only for her heart to shatter into a thousand pieces when she finds out that he has turned himself into the police. Once again, her dreams of living in safety with him and Raoul have been destroyed. The use of Charles Aznavour's wistful ballad "Hier Encore" as the soundtrack, as well as the thematically-appropriate Pensieve Flashback of Claire and Assane as optimistic teenagers meeting at the same train station, only adds to the effect.
The scene is equally sad from Benjamin's perspective: he's finally been released from an excruciating stay in prison, but is fully aware that his arriving in Assane's place will be a huge disappointment to Claire and Raoul. One of the final shots in Part 3 is a closeup on his Thousand-Yard Stare as Claire sobs into his chest.
