It's an interesting notion that I hadn't thought of before, and I tend to agree with what's been stated so far.
Perhaps RTJulia is mostly good--although she's done several evil deeds--specifically because Barnabas, the object of her unrequited love, is similar: capable or great harm, but in a constant struggle to stay good.
If her Barnabas had gone completely to the dark side, become truly evil, would Julia have followed him? I think the evidence from early on in the series is Yes, if Barnabas is evil, Julia would be too...