I can't decide it Ruth is more upset because she is a true Wilford believer or she is angry she wasn't let in on the secret all these years, and feels like a fool for not figuring it out.
Honestly, Melanie's reasoning for leaving without Wilford (I still think he's in a drawer, though) was a good one that made me feel a lot of sympathy for Melanie. She did all the work to actually make Snowpiercer into a rescue mission for humanity when he just wanted to live the high life off the ticket sales with what years he had left.
I think 7 years really is about the breaking point where you have to come to some sort of dialogue about shared resources. Things like "I paid for season passes to the sauna" are starting to look ridiculous. Plus, there is a generation of those (all classes) who were underage when they came aboard and aren't going to cling to pre-apocalypse values. A more benevolent Wilford....er, Melanie...should have been starting (slowly, not too fast) an integration plan based on skills or something. Healthcare, education, & basic nutrition for the Tailie children at the least. Slowly doing some minor redistribution and human rights advances if not because it's right, then because it's the way to avoid a bloody revolution on your hands. Sort of a Snowpiercer New Deal. As mentioned upthread it really was bad writing not to explain or hint why Tailies were allowed on in the first place--even if the initial reason was diabolical on the part of train security. It almost feels like we're missing something.