There's a lot going on here. And stuff that STARTED like 60 chapters ago, it's been building for a long fucking time.
No one has been honest with Shingo, which was probably all he ever wanted.
Kunitachi was saying whatever he believed Shingo needed to hear, going on to say that he believes Shingo hates him. Oof.
Kirishima was someone who was honest with him- Telling him to live up to his own self-worth and to find that outside of his Father, That his obsession with animals/animal corpses wasn't the same as being a serial killer. Stuff like that, to relate and try to understand him. But Kirishima didn't understand Shingo, just in a different manner to Kunitachi.
To begin with I don't believe that Shingo is necessarily obsessed with animal corpses like he'd said. I think it's the opposite.
Shingo loves animals
Recall the previously (chapter 117), Shingo claimed that he was checking on the Swallow to see if it turned into a skeleton.
Shingo was lying. He told a half-truth. The truth was that a cat had dug up the swallow - leading Shingo to be forced to bury it again.
But Shingo couldn't bring himself to admit that he was weak, or interfering with the natural process of wildlife. So he lied.
When Shingo speaks with Kirishima again (chapter 170), the dolphin bones were NOT his priority, Shingo doesn't want to see that.
Why? Because Shingo tells Kirishima that he was amazed when he saw a whale (which is obviously not a corpse).
Kirishima goes on to talk and finally resonate with Shingo, when saying that he wants to return home to his friends. Even if it means doing it by himself.
I'd bet that Shingo finds such companionship in animals because of his lack of human relationships, or relates to them more so because no one understands them.
Shingo tries to speak with Kunitachi about returning and Kunitachi pretty much ignores him - leading to Shingo making that awful 'I hope your surgery fails' remark.
So all of this wraps back around to this moment.
Kunitachi fails to understand that Shingo's remark about the surgery was actually aimed at Kunitachi himself as a cry for help. It wasn't supposed to be taken literally in any sense.
Hence when he actually fails, and Shingo hears his Father broke down - it fucks him up because he feels like his words were responsible.
Shingo sees an animal going to die, and tries to save it. Because, again, he loves animals - he can't bring himself to let it die, or perhaps because of his guilt in the earlier situation.
So when Kirishima says "We can see the Dolphin bones", it's actually the final straw for Shingo. That's absolutely NOT something he wants to see.
At this point Shingo accepts that no one understands him. And jumps.
