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Tear Jerker: Tower of God
During the second part of the Hide-and-Seek test, Ho denies ever being Baam's friend and proceeds to commit suicide in front of him and the other regulars (and Quant).
The omake in which Wangnan visits a Chinese restaurant and orders some noodles, only to be reminded of Nia and break down into tears.
Rachel betraying Baam. The utter look of shock on Baam's face sells it, especially since Baam sincerely looked up to her and promised to help her climb the Tower. The aftermath counts too.
"Become the fence that protects Rachel. Always help her, be kind to her, and pretend to be "friends." But remember. Never get attached to her. Don't ever think of her as your friend. So that you can ignore her cries of pain one day. Despise her. Because she is the woman that killed my most precious friend."
How much Baam has changed.
Baam: (Pre-timeskip) Isn't there...a way to make everyone happy? Not a method like this...
Baam: (Post-timeskip, he chillingly answers his own question) There is no way everyone can be happy. It's only possible that someone will be happy.
Chapter 51 takes it farther; not only did Xia Xia force Baam to turn his back on one of his best friends, but she blew up the Hand of Arlen anyways, and almost kills Koon. Meanwhile, Michael killed Gyetang in a borderline torture session, and Edin Dan gets stabbed by Rachel and looks to be dead.
It's learned in a later chapter that while Dan lived, he will never be able to run as fast as he once could. Rachel made sure to stab him in the legs to take away his amazing speed. Dan says that Rachel left him alive on purpose so that he would lose hope and know what it was like to be stuck in a wheelchair like Rachel was. The whole things is just a cruel, downright tearjerker but does have some hope when Dan refuses to give up and asks Koon to let him on his team for the Workshop Battle..
Viole crying over Koon's supposed death. To a lesser extent, Rachel asking what she did wrong - it's becoming less and less clear if she's outright evil or just very misguided.
In chapter 145 Baam outright says that he's already lost his will to live.
Worse than that, the only thing that keeps him going is making sure everyone is safe. And because he doesn't care about his own life anymore, he'll do it no matter the cost to himself. This is especially painful when you realize that he was forced to drive away his two closest friends and realize that there is no longer a way to go back to being just "Baam".
Viole: Even if I end up broken, if I can protect everyone else, I will do it. If I can do that, it doesn't matter if I break beyond repair since I already lost my reason to live.