This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Until Death Do Us Part
In Chapter 61 Kilo and Sierra's brief conversation in the hospital.
Finding out Luna's identity as well as her very gruesome death, which causes Mamoru's blindness.
Detective Genda's death in Chapter 168 is nothing short of heartbreaking.
Chapter 202. Dear God, Chapter 202. Haruka finally is able to tell Mamoru that she loves him and doesn't want to live without him but surrenders herself to Zasid Turus in order to spare his life. For the first time in the series, Mamoru breaks down into genuine rage and sorrow in his failure to protect Haruka and even doubts his skills in the sword since he couldn't protect anyone with it.