Tear Jerker / Handplates

• The arc where Sans and Papyrus started fighting for the first time after Papyrus saved Gaster from dusting. Even Gaster gets disconcerted about it, just expecting them to get over it and make up and unsure how to take it when they... don't.
• The scenes immediately after Sans and Papyrus make up from their fight, and Sans completely gives up any hope of getting away from Gaster. Gaster notices his listlessness and asks him how he's feeling.
  Sans: i don't feel anything.
• A flashback to Asgore shortly after Toriel left shows how deeply depressed he was, having lost all of his family at once. He's desperate not to be left alone by Gaster—the only thing close to family for him at this point—not even for him to go get a glass of water. And he loses Gaster, too.
• For those who find Gaster sympathetic, the flashback comic where he talks himself into beginning his horrific experiments in the first place by telling himself that he is a selfish, cold, horrible person with no true emotional attachments who just hasn't had the chance to reach his full capacity for cruelty yet (and therefore the only person capable of doing such a thing.) Then he curls up and hides under his lab coat.
  Gaster: It may cost me my life. But I never earned it in the first place.
• A bit of an overarching thing, but the fact that Sans and Papyrus still are very affected by the trauma that Gaster put them through, even though they can't remember him. Sans has serious self esteem issues, as he thinks he doesn't "matter" as much as other people do. Papyrus tries to pretend how great and well-liked he is, and doesn't want to feel dumb, acting full of himself so he can seem happier than he is. Both of them reacted noticeably confused to the idea that blue magic is inherently harmless because Gaster traumatized them so badly they refused to use it. It's a tragic case of how even when someone blocks out painful memories, they can still be deeply affected by them.
• The arc where Sans and Papyrus, after having escaped and lost their memories of Gaster, try to figure out their past and eventually come across Gaster's lab again. They search the entirety of True Lab, looking for a hint about where they came from or something to spark the things they've forgotten, but they get nothing. Papyrus eventually snaps and throws a fit (as well as some of the actual furniture) before ranting about how frustrating it is that he can't figure it out and admitting that it makes him feel dumb.
  • The next comic after that one makes it even worse, with Papyrus wondering who abandoned them and why they weren't good enough for them. Especially harsh since the reader knows that Papyrus and Sans probably wouldn't feel better at all if they were to remember the truth.
• Many fans claim that the Mercyplates AUs are the happier, fluffier side of Handplates, but Zarla's made a few speculative comics that make them just as sad as the original ones. One particular piece has Zarla speculate that if Gaster were to quit when the two were babies and raise them by himself, he would eventually start doting on Sans while neglecting Papyrus because of how much obviously smarter Sans is, leading to Sans feeling pressured and uncomfortable and Papyrus desperate for affection.
• The scene of Alphys in Waterfall, contemplating suicide after the creation of the Amalgamates. Not only are her thoughts clearly paralleling Gaster's from earlier in the comic, Gaster can clearly be seen in her shadow, and he seems to be calling her name, though she can't hear or see him. At one point, his arms can be seen around her in the water. For as unsettling as his presence is, it seems as if he's trying to comfort his former friend and colleague, but can't.
• Sans's slow sinking into the Despair Event Horizon after realizing the Awful Truth about the world... And that Papyrus's line stops before his does.
• The Genocide Route is both this and Nightmare Fuel, especially the Sans fight, where Sans manage to drive the Player into a Villainous BSoD that makes them cry Tears of Remorse.
• Papyrus's death in the Genocide Run is sad in its understatedness. We get Papyrus telling himself to "be brave" and walking into the blizzard...and then Sans stood over his empty clothes, face away from the camera, monotonously saying "we all make choices."