I'm skeptical on the issue of the order having supposedly "fallen to the dark side", but to some extent I think it's an issue of semantics. At any rate, I don't think Barriss should be taken as symptomatic of the whole order, any more than, say, Sora Bulq.
Maybe not "fallen" to the darkside but consumed by it, at least according to Yoda. I'd say Barris's fall was precisely a symptom of this slide. She felt the shift all around her and didn't understand how nobody else could feel it. In a way she was more switched on then even Yoda, it's just her youth and inexperience led her to lash out at the order she felt had betrayed it's core principles and in the process, fell even further herself.
I think Lucas mentioned somewhere that the Dark Side of the force is not a passive thing, but an almost cancerous presence. That once it has it's hooks in someone it'll actively consume them, twisting every good intention towards evil.