You already got the answer to that question back in January over on the Villains Wikia. The short answer is that there is nothing in the movie of Bellwether's life before Judy meets her, so we have no reference as to when she started acting this way.
But even if she was a good person that is now bad, that does not automatically mean she was a hero before. She could have been just a good person and nothing else.
That's been corrected now. Sometimes things like this slip through and we fix them later.
"Heroes turned to the dark side" is a category that gets misused a lot and the reason is because most of the time when it's used, people think "They're bad now. That means they used to be good, so therefore they were a hero that was turned to the dark side." Just like with Bellwether, we have to see how they were good, see how they were a hero and see how they were "turned" to become bad before they qualify. If we don't see the before, we can't say they were a hero.
I covered this a blog, also on the Villains Wikia.
I Did It.
What do you think?