I really don’t see where the people saying the story was “preachy” are coming from. My first playthrough was a neutral route; I didn’t kill any major characters, but I killed a decent amount of normal monsters (I also did the “accidentally kill Toriel then reset” thing) and the only dialogue that even resembled preachiness was from Flowey, an antagonist who tries to make you second guess yourself regardless of what you do. Then at the end of the run, Flowey says something along the lines of “you made it out, but this isn’t really a satisfying ending… I wonder what would happen if you didn’t kill anything?”
Aside from that, the only times anyone tries to tell you “you shouldn’t have done that” are when you either kill a major character who wasn’t trying to kill you (Papyrus), or you’re in a genocide run. Given that you’re never allowed to spare anything or run away from a single fight, some early enemies are immediately spare-able, and the first area in particular requires you to walk in place for several minutes at a time for each of the last few encounters, entering and completing a genocide run is something you’d have to go far out of your way to do. And if you ever abort the run partway through and go back to neutral, everyone more or less forgets you were in genocide in the first place.
My guess is the people complaining about it being too preachy are either so thin-skinned that they can’t stand the idea of a game not patting them on the back for everything they do, or they only watched someone else do the Sans fight (the only fight that’s after the point of no return in genocide) and thought “the mean skeleton man hurt my feelings i cant play this game”.
Look, if the game really didn’t want you to do a genocide run, it probably wouldn’t be there in the first place.