Rasha Mah Hu Omer? How do we know this is the rasha?

What is it about the wicked son's question that marks it as the question of the wicked son? The standard haggadah text makes the question מָה הָעֲבֹדָה הַזֹּאת לָכֶם all about that last modifier, lachem velo lo. Though when contrasted with the wise son's question, it is momentarily problematic, in that he also asks מה העדות והחוקים והמשפטים אשר צווה ה' אלקינו אתכם, but this can be summarily dismissed when we note the likelihood that they were working off a different girsa of the pasuk, which had otanu rather than etchem. (See Yerushalmi Pesachim 70b. There have been many unnecessary and incorrect divrei Torah based on this, but they may be useful to derive and convey an important message, even if this was not the message which was originally intended to be conveyed.) There are other differences, of course, one important one being that the question הָעֲבֹדָה rather than העדות והחוקים והמשפטים implies that it is bothersome, which Yerushalmi stresses as מה הטורח הזה שאתם מטריחין עלינו בכל שנה ושנה. It is possible, and indeed the case, that the Torah's response to a particular son is not necessarily offered to that son. Thus, the response is baavur zeh, which occurs elsewhere. The pesukim which put up the "wicked son's" response are in Shemot 12:

כא וַיִּקְרָא מֹשֶׁה לְכָל-זִקְנֵי יִשְׂרָאֵל, וַיֹּאמֶר אֲלֵהֶם: מִשְׁכוּ, וּקְחוּ לָכֶם צֹאן לְמִשְׁפְּחֹתֵיכֶם--וְשַׁחֲטוּ הַפָּסַח.	21 Then Moses called for all the elders of Israel, and said unto them: 'Draw out, and take you lambs according to your families, and kill the passover lamb.
כב וּלְקַחְתֶּם אֲגֻדַּת אֵזוֹב, וּטְבַלְתֶּם בַּדָּם אֲשֶׁר-בַּסַּף, וְהִגַּעְתֶּם אֶל-הַמַּשְׁקוֹף וְאֶל-שְׁתֵּי הַמְּזוּזֹת, מִן-הַדָּם אֲשֶׁר בַּסָּף; וְאַתֶּם, לֹא תֵצְאוּ אִישׁ מִפֶּתַח-בֵּיתוֹ--עַד-בֹּקֶר.	22 And ye shall take a bunch of hyssop, and dip it in the blood that is in the basin, and strike the lintel and the two side-posts with the blood that is in the basin; and none of you shall go out of the door of his house until the morning.
כג וְעָבַר ה, לִנְגֹּף אֶת-מִצְרַיִם, וְרָאָה אֶת-הַדָּם עַל-הַמַּשְׁקוֹף, וְעַל שְׁתֵּי הַמְּזוּזֹת; וּפָסַח ה, עַל-הַפֶּתַח, וְלֹא יִתֵּן הַמַּשְׁחִית, לָבֹא אֶל-בָּתֵּיכֶם לִנְגֹּף.	23 For the LORD will pass through to smite the Egyptians; and when He seeth the blood upon the lintel, and on the two side-posts, the LORD will pass over the door, and will not suffer the destroyer to come in unto your houses to smite you.
כד וּשְׁמַרְתֶּם, אֶת-הַדָּבָר הַזֶּה, לְחָק-לְךָ וּלְבָנֶיךָ, עַד-עוֹלָם.	24 And ye shall observe this thing for an ordinance to thee and to thy sons for ever.
כה וְהָיָה כִּי-תָבֹאוּ אֶל-הָאָרֶץ, אֲשֶׁר יִתֵּן ה לָכֶם--כַּאֲשֶׁר דִּבֵּר; וּשְׁמַרְתֶּם, אֶת-הָעֲבֹדָה הַזֹּאת.	25 And it shall come to pass, when ye be come to the land which the LORD will give you, according as He hath promised, that ye shall keep this service.
כו וְהָיָה, כִּי-יֹאמְרוּ אֲלֵיכֶם בְּנֵיכֶם: מָה הָעֲבֹדָה הַזֹּאת, לָכֶם.	26 And it shall come to pass, when your children shall say unto you: What mean ye by this service?
כז וַאֲמַרְתֶּם זֶבַח-פֶּסַח הוּא לַה, אֲשֶׁר פָּסַח עַל-בָּתֵּי בְנֵי-יִשְׂרָאֵל בְּמִצְרַיִם, בְּנָגְפּוֹ אֶת-מִצְרַיִם, וְאֶת-בָּתֵּינוּ הִצִּיל; וַיִּקֹּד הָעָם, וַיִּשְׁתַּחֲווּ.	27 that ye shall say: It is the sacrifice of the LORD'S passover, for that He passed over the houses of the children of Israel in Egypt, when He smote the Egyptians, and delivered our houses.' And the people bowed the head and worshipped.
כח וַיֵּלְכוּ וַיַּעֲשׂוּ, בְּנֵי יִשְׂרָאֵל: כַּאֲשֶׁר צִוָּה ה אֶת-מֹשֶׁה וְאַהֲרֹן, כֵּן עָשׂוּ. {ס}	28 And the children of Israel went and did so; as the LORD had commanded Moses and Aaron, so did they. {S}

There definitely are the above cues to interpret this on a midrashic level as the question of the wicked son. (On a peshat level, Moshe is telling them great news about the future, that they will be redeemed, brought to the land of Israel, and will have a peaceful future many generations down the line in which a son will have to ask the nature and reason for the sacrifice.)

But what about the response -- that since he takes himself out of the community, you tell him that if he were not there, he would not have been redeemed? This certainly can be just a derasha on lachem followed by the response culled from elsewhere, that one can respond baavur zeh asa Hashem li, which is a normal response, but with certain words stressed to form a meaningful retort. But despite the fact that it is not the response in the haggada, perhaps the local response to this question in Shemot 12 can shed light on the type of response offered. After all, the answer is that the purpose of the sacrifice, the zevach Pesach in Mitzrayim, is to identify oneself as an Israelite. And in makkat bechorot, Hashem skipped over all the Israelite houses which in which people chose to participate in this and declare that they were Israelites. But any house not so marked would presumably have been smitten together with the houses of the Egyptians. And this, says the pesukim, is the purpose and function of the zevach Pesach. From the response, we can then extrapolate the nature of the question.

And indeed, not because of merit or lack thereof would he not have been saved had he been present. Rather, if he had been there and not joined the community in this, and identified as part of the community, he would not have been saved, for there would not have been the blood of the Paschal offering on the mashkof and the two mezuzot.

There might be some homiletic message we can derive from this, if we wished to, about the importance of participating in some ritual just to participate as part of the community.