The death of Bulat. Being the Awesome Mentor he was, and his Dying Moment of Awesome, passing the torch, and his final moments being being an Expy/Homage of Kamina. Since it's one of the few deaths that isn't preceded by a flashback of his youth, you can even hold some hope for him... but sadly, he was one of the first characters to die.
When Bors dies, it's because he came across a girl who'd been injured in the crossfire of his battle with Night Raid and kindly offers to treat her wounds. Only it turns out that it's Chelsea in disguise. After giving Bors a swift but fatal injury, she explains how he burned down her entire village because they were suspected of harboring rebels. Bors admits that he always knew karma would come to get him because of all the awful things he'd done, but that doesn't stop him from having a vision of when Esdese convinced him to reveal his face to the other members of Jaegers, and for them to not treat him any differently because of it, followed by a flashback to the moment his daughter was born, and lastly by a vision of his wife and daughter at their home.
To make this even worse you get to see them at his grave along with a grieving Wave/Ran. Let's not talk about what happens at that place later on. It's too bloody horrible.
Extra Chapter 23.5 tells the story of three girls named Fal, Air and Luna, whose parents sold them to a man in the capital because heavy taxation had left them destitute. They look forward to having a kind master, which their master at first seems to be. Then he sells them to some of the most depraved people imaginable. Fal dies of the ensuing gory torture, and Luna commits suicide after having her eyes cut out. Air survives her disgusting treatment and hires Night Raid to take revenge. They do so, and once that's done, she kills herself with poison to join her friends. The icing on the despair cake is that she does this on the edge of a crowded street, and no one so much as looks at her.Damn.
Chapter 56: The sad ending image of a beat up Tatsumi talking to a non responsive Mine.