What is the significance of "Yesh Ra'ah"?
Refer to 5:12:1:3 and the note there.
Why does it say "v'Rabah Hi Al ha'Adam"?
Rashi: This applies to many people.
Ibn Ezra: 'Rav' has four meanings. (a) It is the opposite of few - "la'Rav Tarbeh Nachalaso" (Bamidbar 33:54). (b) Quarrel - "Rav Es Yotzero" (Yeshayah 45:1), which is like Riv (Bereishis 13:7). (c) Shooting arrows - "Roveh Kashas" (Bereishis 21:20), "Hashmi'u El Bavel Rabim [Kol Dorechei Keshes" (Yirmeyah 50:29). The grammatical form [of Rabim] is like Dalim. (d) Great 1 - "Moshi'a va'Rav v'Hitzikam" (Yeshayah 19:20), "Al Kol Rav Beiso" (Esther 1:8). Here, it is feminine (Rabah).
R. Avigdor: This is swindlers' intents, e.g. [a seller] who mixes water with wine or oil [so he will take more money for a larger volume].
Seforno: This evil usually occurs to people.
Metzudas David: It pains man very much.