I am so sorry your Birthdad's Mum is reacting in this way. Totally uncalled for, and unnecessary, I think.
But something you wrote in your post hit me, and it is something that you have to remember first and foremost...
"Seriously, she's never met me, I was raised by completely different people (Barbara hates my bmom), and you'd think (or at least HOPE) that an adult would act better. She knows nothing about me whatsoever and thinks I'm a horrible person/monster, and I hafta say, it's just the teeniest bit hurtful. So I'm venting. Thanks for reading"
She does not know you, but she thinks you are a horrible person? Judging without knowing...you are correct, an adult should behave better, maybe it is her own guilt and issues that are the problem here....
You know what, it is her loss! You continue to have relationships with those that do welcome you and not judge you, if she comes around, great, if she doesn't, then it is because of HER issues, not anything to do with you.
Collette