This has been my exact view on what's happening, and I thought it was obvious that it was intentional on the part of the writers, but apparently not.

Shauna was who we were rooting for in season 1/2; the bedraggled underdog that couldn't catch a break. Now we're seeing that she might be the worst out of the bunch as we get more information. Honestly her adult behavior in season 1 & 2 was not so subtly hinting to her possibly being a terrible person. A lot of people probably made excuses for her behavior because she was the person they hoped would "succeed" or whatever. It's amazing the kind of behavior we're willing to justify in others when we want to see them win.