We don't need to have a first hand knowledge of evil.
1. God put the "Tree of the Knowledge of Good and Evil" in the garden despite knowing that Adam and Eve would disobey him, because He has given them free choice.
2. God could give that knowledge of good and evil to Adam and Eve without them having a first hand knowledge of it.
3. Adam and Eve doubted God, therefore went to grab that knowledge instead of trusting in Him and receive that knowledge.