Showjoe, it was because they had pessel micha with them. They actually carried it into the Yam Suf where they were being saved and the Mitzrim were being killed. The complaint was at the juxtaposition of these being saved and those being drowned- same time, same place, seemingly same type of people.
Your idea makes no sense because the neis of splitting the sea was done in such a way that the yabasha was divided into 12 by The One who made the Neiss, not by the people walking through. But if you bring me a source instead of an “I heard”, I’ll give you a pass.
IvduEs, don’t worry, you can keep on loving everybody Ad Bi’as Goel, and (hopefully) beyond. Remember- “Yitamu Chata’im…”, Not Chotim. Maintaining individuality in our path to Avodas Hashem, according to the derech of our Mesora, doesn’t mean we hate anybody. Not sure what you’re having trouble with.