In Sarah's Key, how did Sarah react to the roundup?

In the novel Sarah's Key by Tatiana de Rosnay, the character of Sarah Starzynski is plagued by intense regret over the events of the roundup. When it becomes clear that police are coming to raid their home, Sarah locks her younger brother, Michel, into a cupboard in their bedroom. While Michel suggested the action, Sarah followed through and put the key to the closet in her pocket.
Sarah and her parents are taken by the police and ultimately locked up themselves in a refugee camp and, later, Auschwitz. Sarah is terrified of being taken away and vows to return to her brother to unlock him from the cupboard. While all of the occupants of the camp are desperate to escape, Sarah knows she has limited time and that if she cannot get away, her brother won't be safe. While she ultimately escapes and makes a harrowing journey home, she does not arrive in time, and she discovers her brother's body in the cupboard.
In the immediate aftermath of the raid, Sarah is regretful and afraid for both her life and, more importantly, Michel's. After she discovers his body, her remorse grows, and she begins to spiral into a lifetime of depression. Sarah ultimately takes her own life because she is unable to live with the pain and shame of her brother's death. She was just a child, faced with an impossible choice, but once she learns he has died, she knows she made the wrong decision, and that reality is too much for her to handle.