What made the Rasha(wicked son) a Rasha?It is interesting in that it is part of a general trend nowadays to bolster Orthopraxy rather than Orthodoxy as that which is required of a religious Jew, a trend also present in how many people deal with Marc Shapiro's The Limits of Orthodox Theology.
When we read about the 4 sons in the Haggadah, three are somewhat self explanatory: the wise son, the simple, the one who doesn't know how to ask. But labelling one of the sons as wicked begs the question: Why is he wicked? what did he do or say or believe to deserve the title?
One start at the answer is in the response "l'fi sh'hotze et azmo min ha clal kafar ba'ekar" because he removed himself from the community he violated a basic belief. What was that basic belief? one of the Rambam's 13? Go on to the next part "ilu haya sham lo haya nigal" If he had been in Eygpt, he would not have been redeemed. Who didn't get redeemed? The midrash tells us that many Jews perished during the plague of darkness, those that were not worthy of being redeemed. It seems that they were in the catagory of Rasha as well. What was their misdeed? The children of Israel had been commanded to take a lamb in preparation of the korban pesach(pascal sacrifice). My father in law quoting a 10th century midrash says that those who did not do so died during the plague of darkness. Those who were not redeemed did not perform an act which would have symbolized their belief that redemption was around the corner. They didn't actually have to believe that the redemption was near, only do an act. Those that believed, but didn't do an act, also died. As is common, it is the action that is important, belief is good, but not essential. A similar use of the word Rasha is when Moshe sees two Jews fighting and says "Rasha, lama ta'ke rayecha" bad man(rasha), why are you hitting your neighbor. Again, rasha refers to someone who is doing an act, rather than a belief(or lack or belief).
We see that Rasha is someone whose ACTIONS take him away from the community, and who does not join with the community in actions that reflect belief. However, the Rasha does not earn the name simply with beliefs.
(thanks to Irina for asking the question and making me think about it, and thanks to my f and fil who always have answers to such questions)
Now, on to the subject matter. How did he remove himself from the general community? With his language. The haggadah cites his question - Mah HaAvodah HaZot Lachem. To you, plural. And then it makes a drasha - Lachem, and not Lo, to him. He should have Lanu said rather than Lachem. That is how he separates himself from the community. And either distancing yourself from the community, or not joining them in their approach to serving God, and thus denying God, is considered kafar baIkar - denying a fundamental. This may or may not be parallel to one of the Rambam's Ikarei Emuna, but most likely they were not thinking in terms of Rambam when they said this, and we should not twist and stretch the plain sense of the haggadah to address this "question."
However, the main point is that the problem with the Rasha' is his choice of language. We see exactly the same thing in Bavli Brachot. The Mishna, towards the end of Brachot, gives different formulas for mezuman depending on if there is a mezuman with the leader or not. If there are three, he says Nevarech. If there are four, and thus three even without him, he says (or may say) Barchu. The difference is that Nevarech includes himself in the invitation to bless, while Barchu does not include him. This is the same grammatical difference as between Lachem and Lanu. On this Mishna, we have Shmuel's statement that he never removes himself from the klal, that is the same language as used in the haggadah. This means that even when there are four, Shmuel would say Nevarech, and so indeed is our custom.
Pashut peshat is that he is a Rasha because of his attitude and his words reflect this attitude. (As an aside, the use of this language by the Chacham is not a problem, for in the yerushalmi or Tosefta, we see the original girsa of his question. and the pasuk, which has otanu rather than etchem.) The father's response is not meant to elaborate upon the definition of removing himself from the klal. It is parallel. Just as the Rasha removed himself from the community by saying "you," not including himself. the father responds in a way that may be darshened to exclude his son, who by virtue of being wicked, would not have been redeemed.
But it is attitude, as opposed to any action, which makes the son wicked, in this instance.
Another point. Dilbert mentions that in the case of the one Jew hitting his fellow:
A similar use of the word Rasha is when Moshe sees two Jews fighting and says "Rasha, lama ta'ke rayecha" bad man(rasha), why are you hitting your neighbor. Again, rasha refers to someone who is doing an act, rather than a belief(or lack or belief).In fact, if you look carefully at the Midrash on this pasuk, you see how Chazal deal with it. There is a linguistic point being made. Takeh is the imperfect (future) tense, not present tense. As a result, it literally means "why will you (in the future) hit your neighbor?" The Midrash explicitly takes this to mean that even if you raise your hand with intent to hit your neighbor in the near future, you are considered a Rasha. There is still the action of raising a hand, but it is a far cry from how Dilbert is trying to use it, that only if you are engaged in hitting at the moment are you a Rasha. Intent has a lot more importance here.