Undertale is designed to naturally guide you through all 3 routes - Neutral, Pacifist, then (optionally) Genocide.

This is why some of the sparing mechanics can be so obtuse, especially with Toriel, who you don’t hear about until the Asgore fight when you’re like “oh damn I killed his wife”. There’s even a Froggit NPC in the Ruins who says you can spare enemies at low health, which could trick you into trying that with Toriel - doing so doesn’t work as your last hit does a lot of damage.

The game wants you to earn some EXP to increase LOVE until you learn what the acronyms mean.

If you spared Flowey at the end of a Neutral run, he comes back and tells you he won’t kill Asgore if you can get there without killing anyone, basically telling you how to do the Pacifist route.

Then, when you re-fight Toriel after killing her once, the Talk option explicitly mentions that you’ve killed her, and hints that you can “show mercy without running away”, making it easier to do a Pacifist run on your second go.

Characters will also mention feelings of deja vu on a second playthrough - Toriel has “a feeling” about your pie choice and Papyrus thinks you look familiar. And you won’t have to re-fight Omega Flowey.

Of course, it’s possible to figure out all the spare mechanics on your first try, which is why fighting Omega Flowey once and seeing the Pacifist phonecall is mandatory to unlock the final sidequest needed for True Pacifist.

And if you happen to boot the game up again after the True Pacifist ending, Flowey tells you that you’re the only threat left, with the power to reset everything, hinting at Genocide.

There’s some extra wrinkles, like how you can just reload your save if you killed Toriel and try again (which Flowey calls you out for), or how doing Genocide first changes things, but I think this was the main intended path - to do a run before you know you’ll be judged, then decide if you wanna be nice or evil with that in mind.

TL;DR: Get it twisted, you’re meant to kill Toriel once.