"And when your son will ask you, 'What is this' and you will answer him that it was with a mighty hand that Hashem took us out of Egypt, from the house of bondage" (Shemos 13, 14). Rashi notes that in four different places the Torah mentions the son's question with regard to the exodus from Mitzrayim. He explains that the Torah is referring to four different personalities: the wise, the wicked, the simple and the one who doesn't even know how to ask.
Rav Wolbe (Shiurei Chumash) comments that if the Torah felt it imperative to write four separate pesukim to parallel the four different sons, it implies that every child must be spoken to in a language that he can understand. Even the wicked son must be answered with a response that is hand tailored to his personality.
Additionally, Rav Wolbe points out that the wickedness of the wicked son lies in the way he phrases his question [regarding the korban Pesach], "What is this service for you?" As we say in the Haggadda, "for you and not for him"- he inquires but does not identify with the topic of his question. This is a "wickedness" that many of us also possess; we learn things but they may not necessarily have any impact on our personal lives. With regard to the spies who were sent to Eretz Yisroel and returned with a derogatory report, Chazal tell us, "These wicked men saw [what happened to Miriam who spoke derogatorily about Moshe] and they did not derive musssarfromit." Mussar is the ability to integrate the lessons learned, into our day to day lives. One who does not derive mussar from what he learns - no matter how great his stature - is considered by Chazal to be wicked in this respect.
The study of mussar is not simply opening a Mesilas Yesharim and perusing what is written therein. When Miriam was afflicted with tzora'as, everyone certainly sat and learned through the sefer Chofetz Chaim. The problem was that their learning didn't impact the way they acted. True study of mussaris taking a line of the mesilas yesharim, comparing it with our personality, and in the event that it doesn't match up, working to change to meet the standard of the Mesilas Yesharim!