When I first knew him your father was already a great pilot. But I was amazed on how strong the force was with him. I took it upon myself to train him as a Jedi. I though that perhaps I could instruct him just as Yoda, I was wrong.
~ Obi-Wan Kenobi on how Anakin Skywalker betrayed him

Heroes who are masters and have students/servants/apprentices/disciples who are villains. Many of these masters are most likely to be naive and therefore not know that the villains who serve them are plotting to betray or usurp them.