Gentaro being brainwashed and convinced by Ophiuchus that his friends hate one another in III-Genesis, going as far as to cry out in terror at what he's witnessing.
Gentaro: "Stop it! I CAN'T TAKE IT! STOP IT NOW!!!"
What makes it worse is that Ophiuchus is lying through his teeth. The next chapter, the Kamen Rider Club is not only mourning his death but also blaming themselves for not doing anything to save him.
Or how about the fact that this is Gentaro, the one who'd befriend everyone, being subjugated to this? As if dying wasn't enough...
Ryusei's fate. He truly thought he was going to save Jiro by killing Gentaro, as shown in the prequel Who Decides?. He realizes too late that Gentaro was resurrected and hates his guts, the Kamen Rider Club hate his guts more, Jiro dies, and then later he's brainwashed and configured into a cyborg. He's not getting a happy ending, not at all.
Worse? The last image he sees before Tachibana begins his configuration? Gentaro smiling and extending a hand toward him.
JK's backstory, as revealed in SplitxEnd. Why is he a Knowledge Broker and what was the incident that happened to his father? It turns out he had a little sister named Keiko who was killed trying to save her brother from a sociopath caretaker. The event traumatized him and he became a Knowledge Broker so that he knew more about people and so he didn't experience an attack like that ever again.
Mei Shirakawa's life. To summarize, her parents are absolute jerks who don't recognize the problems she has in school involving Yamada Tatsumori, the two boys she was seen in the show were paid by Yamada himself to be her friends, she saw Ryusei kill Gentaro, she learned Jiro died and her mother left her at the hospital, learns that her parents were jerks because she failed some special ritual..., the list goes on and on. Is there any reason she doesn't become a Serpent-bearer because of this?
Tears to Shed, just like the song it's titled from, is Gentaro's struggles with his humanity. He's seen crying in the first chapter, wishing to be with people despite the fact that Ophiuchus has brainwashed him that friendship has hurt him.