"I hate Lilly, and I... I hate you more than anyone...!" Beware The Quiet/Nice Ones indeed.note For context, this is from Hanako's Bad Ending, where she violently snaps at Hisao, accusing him of only ever spending time with her because he pitied her and not viewing her as an equal.
Hisao Nakai suffers from a heart defect diagnosed as arrhythmia that, at any moment, can kill him if he's not careful to take his pills and not overexert himself. When his heart starts to act up, it is accompanied in game by a prominent Heartbeat Soundtrack and red vein-y borders around the edges of the screen. It's frightening enough to almost convince you you're about to have a heart attack...
The first time this defect starts. Oh, how sweet! His crush is about to admit her feelings for him! Then, suddenly, thud-bump. THUD-BUMP.
His heart flutters in general. Good lord.
A common theory about the Act 1 Bad Ending is that Hisao didn't just fall to his death, but Kenji actually set the whole thing up, and pushed him off intentionally. This is mainly due to him saying a few scenes earlier that pushing someone off a building would be the ideal way to kill someone. To think that Kenji might have planned the whole thing as a way of eliminating Hisao is somewhat creepy, to say the least.
A case of Adult Fear comes in Act 3, where she is forced to listen in horror as Hisao, the friend she secretly has romantic feelings for, has a heart murmur meters away from her. Due to her blindness, she can't see what's going on, or help him at all.
As pictured above, Hanako's Bad Ending is definitely this. Equal parts shocking, depressing and utterly terrifying, it comes as quite a Player Punch when it arrives. Hanako has a violent outburst of anger towards Hisao when he insists on trying to protect her even though she told him to leave. The scene reveals Hanako's character to be a complete deconstruction of the typical Woobie, and acts as one of the biggest What the Hell, Player? moments in the game. It's a wonder how Hisao didn't had a heart attack during it.
The final CG, which isn't scary so much from the details as it is from the fact that it COMES WITH NO WARNING.
A heavy implication in Rin's Act 4 Neutral Ending gives us some terrifying Fridge Horror: Since her leaving parallels Sae's story about her husband, who committed suicide, and the way she begs Hisao to forget about her after she leaves for Tokyo, gives the hint that she might be planning to take her own life at some point in the near future since Hisao turned out to be unable to connect with and understand her in the way she was hoping he would.
Also from Act 4, Hisao exploding at Rin in frustration. It feels like a Bad Ending in and of itself, only the positions are reversed: Rin is basically the protagonist who screwed up, and you're the one furiously calling her out on it. The fact that you're the one speaking them makes those words hit all the harder.