The demo's final battle with Toriel. Killing her in the semi-bad ending makes her die smiling, telling you to be good one more time, while telling you how strong you've become.
Not that choosing to spare Toriel by not fighting is any less of a Tear Jerker. When your HP gets low enough, her attacks will intentionally start avoiding your soul (to the point it's impossible to get hit by them), and she puts aside her loneliness, fear and doubts to let you have your way. Toriel gives you a Final First Hug, and lets you go.