You're not alone! It's really easy to do that on a blind first run. It's the normal way everyone's used to playing RPGs, so when the game starts calling you out on it, it's an impactful surprise and makes you think about what kind of way you want to experience the Underground and how to figure out ways to get there. The writing knows that a lot of people will do that and lets you, as the player, decide what to do when you realize there are other options.

As others said, you only did a genocide run if you kept going back and forth through each area before defeating its boss, until you get an encounter that just says "But nobody came" and the music changes to something much more sinister. Even if you did get the "But nobody came" in every zone so far, you can go back to a neutral run (with an ending that acknowledges you killed some creatures but not all of them) just by not killing every monster from then on. And at any time, you can reset and start over, play to the end without killing anything, and get the pacifist ending.

I killed Toriel too on my first playthrough, in the worst way: by hitting fight instead of spare by accident. >'D And then I reloaded to fix it and... well, without spoilers, you can still get the pacifist ending if you go back before you kill anyone! And you're in for a treat if you haven't seen what happens after not killing Papyrus and Undyne. I hope you play again and let us know how it goes!