Sabrina supposedly has a free will choice to follow either the Path of the Light or the Path of the Night, because she's a hybrid. Since she has human emotions, and thus some sense of morality, most of the evil stuff witches would be forced to do is hidden away from her. Dealing with demons, giving your soul to the Dark One, loss of free will, human sacrifice and blood rituals, etc, are all hidden from her to try and make the upsides of being on the Path of the Night more attractive than the powerless Path of the Light followers.
Her humanity revolts against those ideas, trying to help people that are being bullied or hurt, and she comes to learn that she'll not only be forbidden from helping people, but will actively be forced to hurt people, even perhaps people she loves or feels sorry for. Since she's already been foretold as being the most powerful witch alive, or even perhaps ever (we're not told exactly what, but it's implied), it's important that the Dark One gets her under control any way possible. Hiding this information from her was a 16 year deception aimed to try and get her to commit to being the Dark Lord's slave.
Basically, she didn't know, because she would have chosen the Path of the Light in a heartbeat, and the Dark One's plans would have been ruined, or at least significantly delayed, since ultimately it was her humanity that leads her to make the decision that she did make. The Dark One was simply trying to use persuasion instead of force, since it was the path of least resistance.