What did Satan get out of deceiving Eve and Adam into sinning?

The devil deceived Eve into eating what was forbidden. She then persuaded Adam to do likewise. After eating, they gained knowledge of good and evil. It is my view that God's true intentions, his omnibenevolence, is presented in the Ten Commandments and the Bible. Basically they are a moral guide to overcome the sin we all inherited from Adam and Eve. But what was Eve's situation before the bite? Did she have knowledge of good and evil? Was she pure still and acting as God meant her to act? Why is gaining knowledge of good and evil in the devil's advantage? Because she had no knowledge of the bad yet? She knew the good neither. Wasn't it a great gift then, the devil gave her with the apple? What gain had the devil to give her knowledge of the good? If Eve had no knowledge of good, how she could act good? By divine instinct? What was in it for the evil devil Satan, disguised as a luring snake? Was this, as the fallen angle, his way to take revenge on God? By letting people know what is bad? At the same time he gave knowledge of the good. Wasn't it better to give Eve knowledge of the bad only?