EULERHOPEPUNK'S PROFILE

Toriel's fight actually treats opening an attack, and letting the bar used for timing run its course as the "spare" command. This actually works well within the narrative too - you show Toriel you're capable of fighting, but you choose not to.

However, once you get Toriel to a position where she starts speaking about how you could stay with her, all of your attacks do critical damage, because she wants to fight just as little as you do. This "choosing not to fight but displaying you know how" mechanic makes it easier to accidentally kill Toriel, since you're spending most of your time in the fight menu rather than the spare menu.

So even if you aren't spamming "spare" like the plague, you're still likely to be tricked into accidentally killing her.