When Frisk takes the downward path in Home's Heart, a cutscene plays where they are ambushed and killed by a silhouetted figure with an axe. Frisk isn't even given a chance to dodge or fight back via a battle scene; their fate is sealed the moment the cutscene starts. And we never get to see what else lies down that passage.
According to the Bake Sale Spider, both of the humans Papyrus adopted were killers. Justine apparently had an entire Genocide run's worth of LOVE by the time she fell (though miraculously never killed a single monster), while Alice fell with zero kills in her past but within a week had killed enough monsters that the population of Old Home was noticeably lower.
"From a human who killed before but never down here to a human that only killed down here, what terrible luck."
Unlike in canon with the original Asgore, everything so far implies that Asgore!Sans has been more willing to kill human children with his own two hands for the sake of gaining enough souls to break the barrier.
Subverted later. It's revealed during Chara's final date that he's as hesitant as canon!Asgore.
Usually outside of when you talk to them, Chara is seen with a perpetual smile on their face. But you reach Undyne's resort and... they're not smiling anymore. It's unnerving, and even ignoring what happened in the phone chat with Asriel, it is a clear sign that something is wrong.
When Chara is telling their story of their life in Mount Ebott. When they start talking about how skeptical they were of the monsters' kindness, their eyes lose their light. It's regained after they talk about their meet with Papyrus behind the door, thankfully.
Toriel mentioned earlier that Chara was barely alive when she "rescued" them from Sans. From what Chara says, Sans had only caught them as they fell and was about to set them down before Toriel came in. Just what happened to them beforehand?
The neutral "No Mercy" ending is this with a side order of Tear Jerker.
In the call, Chara's hair is disheveled, their eyes are without light and their signature blush is gone. Clearly, they have snapped, and with good reason. Thanks to a mistake of identity, the surviving monsters think Chara was the one killing their kind and are forming something akin to a lynch mob against them. Everyone is against them, and whoever that could've helped clear their name or protect them (namely, their family) are dead. And the worst part? Chances are, this is a probable repeat of whatever life they had on the surface prior to falling down Mount Ebott.
If you've killed the bosses and less than twenty other monsters:
Chara:Everyone's waiting back at home. Everyone's waiting to throw rocks at me and make my life a living hell. But I won't let them. I... (smiles) I'll fix the problem. Goodbye and good riddance.
If you've killed the bosses and more than twenty other monsters:
Chara:...I'll be sure to greet you again. You know, when we meet in hell. Race you there!
Undyne:F I N A L L Y. N O M O R E D I S T R A C T I O N S .
After you are cornered by Undyne in the CORE, you both are confronted by "Corinthian", a.k.a. "Reaper Bird". Even the dialogue box is afraid of it.
Undyne throws a spear at it. Not only does it not work, it also causes some rather intimidating dialogue.
Pick a god and pray | You will regret that | You deserve this | Take your last look | Robbit, rip it | You have much to learn
Even as you are both fighting against Corinthian, Undyne is still trying to kill you. Caught between both her and the Amalgamate, Shifty freaks out, starts screaming (which pressures Undyne into retreating) and then regresses into a crying wreck. Fortunately, this is when Corinthian decides to show mercy.
The terrifying implications about what happened to Snowdrake's mother. She is the only amalgamate who is not wandering the Underground. A letter from Snowdrake in Asgore's lab has a single word scribbled on a corner: "Hurry." Snowdrake's father says that Asgore needed more time to help her and that she "wasn't stable." Whatever's going on, we are in for a treat when we reach the True Lab.
In a pacifist run, you meet Sans!Chara in the Hall of Judgment. Chara calmly states that they can see all the lines of data that make up the game before judging you based on your EXP and LOVE. While this moment appears Heartwarming based on how they look so happy and proud of you, it becomes rather worrying when you step away and think about the WHY of the matter. You get the dawning horrifying realization that Chara had spent the whole time fearfully watching those numbers, praying that they will never rise, and if they do, it's not caused by killing any of their family members.