It takes some really serious farming to accidentally find genocide route and then not exit it my mistake. Most players blindly treating Undertale as a regular RPG would simply kill the enemies they encounter (which is not enough to trigger genocide route), maybe regret it along the way, and eventually get a neutral ending plus the hints to try pacifist.

It's clearly the intended progression, since the second playthrough then changes to reflect the fact that the game "remembers" your first one.