1) The dor ha'mabul was punished because וַתִּמָּלֵ֥א הָאָ֖רֶץ חָמָֽס. What is חָמָֽס? The Midrash writes that the issur gezel is when you steal the amount worth a perutah; חָמָֽס is when you steal less than a perutah. Technically, it's not theft. The gemara (BK 62) gives a slightly different definition. Gezel is when you don't pay for what you've taken; חָמָֽס is where you paid, but twisted the seller's arm to force the sale and get what you want. The gemara itseld writes that people think that 'lo tahcmod' only applies if you force someone to sell and then never pay, but if you pay, then you did nothing wrong. True, it's not gezel, it may or may not be lo tachmod, but it is חָמָֽס. The common denominator of both definitions: obeying the technical details of the law but circumventing the intent and spirit of the law.
What's less than a perutah? It's a nothing. What can it hurt to steal it? And in truth, that's what a person is not chayav for doing so. But when everyone starts behaving that way, when תִּמָּלֵ֥א הָאָ֖רֶץ חָמָֽס, society goes down the drain.
R' Moshe Avigdor Amiel writes in Hegyonot El Ami that the punishment of the mabul is midah k'neged midah. What's a single drop of water? Water can even be a positive thing -- nothing can grow or exist without rain. But when you put together all those drops, and you keep it up non stop for days, the world is destroyed.
2) וַיֹּאמְר֞וּ אִ֣ישׁ אֶל־רֵעֵ֗הוּ הָ֚בָה נִלְבְּנָ֣ה לְבֵנִ֔ים וְנִשְׂרְפָ֖ה לִשְׂרֵפָ֑ה וַתְּהִ֨י לָהֶ֤ם הַלְּבֵנָה֙ לְאָ֔בֶן וְהַ֣חֵמָ֔ר הָיָ֥ה לָהֶ֖ם לַחֹֽמֶר
וַיֹּאמְר֞וּ הָ֣בָה׀ נִבְנֶה־לָּ֣נוּ עִ֗יר וּמִגְדָּל֙ וְרֹאשׁ֣וֹ בַשָּׁמַ֔יִם וְנַֽעֲשֶׂה־לָּ֖נוּ שֵׁ֑ם פֶּן־נָפ֖וּץ עַל־פְּנֵ֥י כׇל־הָאָֽרֶץ
The process seems out of order. You would expect the Torah to tell us that the dor ha'palaga wanted to build a city, a migdal, and therefore they began making bricks to accomplish that task. You first need a goal and then you devise the process to get there. Instead, we are told that they made bricks first, and then they came up with a goal and decided to make their migdal.
R' Yosef Chaim Zonnenfeld (quoted in Binah u'Bracha by R' Baruch Sorotzkin) explained that it's not the desire to make a migdal and rebel against G-d which drove the creation of new technology of brick making, but rather the new technology of brick making, which may have been developed with no nefarious purpose in mind (see Seforno), inflated man's ego and drove him to rebellion.
This was R Y C Zonnenfeld's answer to Franz Joseph as to why the more civilization advances, the weaker religious committment seems to grow.
3) Speaking of pesukim out of order, in 6:14 Noach is given the command to build a teivah, עֲשֵׂ֤ה לְךָ֙ תֵּבַ֣ת עֲצֵי־גֹ֔פֶר קִנִּ֖ים תַּֽעֲשֶׂ֣ה אֶת־הַתֵּבָ֑ה, which is followed by a few pesukim in which the dimensions of the ark and how it should be made are spelled out. Only after completing that description, Hashem tells Noach that he is going to bring a flood, וַאֲנִ֗י הִנְנִי֩ מֵבִ֨יא אֶת־הַמַּבּ֥וּל מַ֙יִם֙ עַל־הָאָ֔רֶץ לְשַׁחֵ֣ת כׇּל־בָּשָׂ֗ר אֲשֶׁר־בּוֹ֙ ר֣וּחַ חַיִּ֔ים מִתַּ֖חַת הַשָּׁמָ֑יִם. Shouldn't it have been presented in the reverse order, i.e. first tell Noach there is going to be a flood, and then tell him that he has to make an ark to be saved?
R' Yichiel Michel Feinstein quotes the Rashi on 6:14: הרבה ריוח והצלה לפניו בשבילם.א ולמה הטריחו בבניין זה? כדי שיראוהוב דור המבול עסוק בבניין זהג מאה ועשרים שנה, ושואלים אותו: מה הוא זאת לך, והוא אומר להם: עתיד הקב״ה להביא מבול לעולם, אולי ישובו It was not the flood which was the cause pushing Noach to build an ark to save himself. Aderaba, it was the building of the ark, the warning flag Hashem put up and which was ignored by the dor ha'mabul, which sealed their fate caused the flood.
4) See here and here for posts on the Midrash's comment on "Vayarach Hashem es rei'ach ha'nicho'ach" that Hashem was smelling not just the fragrant odor of Noach's korban, but was also smelling Avraham in the furnace of Nimrod, he was smelling Chananya, Mishael, v'Azarya in their oven, he was smelling the sweet scent of all those who sacrifice their lives al kiddush Hashem. I don't want to repeat everything I wrote before, but b'kitzur, smell is a special sense, the one sense that was uncorrupted by the cheit of Adam haRishon, and therefore is the key to man rehabilitation, his return to Eden, it is the promise of a better future when all seems lost and corrupted. I was thinking about this topic again this week because my SIL was talking about whether odor can be transmitted electronically. Ain beit medrash b'li chiddush, so I want to add just one point to those older posts. The gemara (Zevachim 113) asks how the giant animal called the re'em was saved from the flood. It was so big, says the gemara, that even its head alone could not fit in the teivah. Answers the gemara, it stuck just it's nose in and was able to survive. Rav Ben Tzion Kook writes that even if a person is outside the beis medrash, so long as they are connected to the "reiach haTorah," there is hope. Even if the rest of a creature is underwater, so long as the nose, the kli for the sense of smell, is still in the teivah, there is hope for survival.