Yeah, I agree that her behavior is not normal, but she was abused and lost her innocence at a very young age, so that must have contributed to it. Still, I have this uncanny feeling that the adults have put the words in her mouth -- like how she knew exactly how her dad used to abuse her, and how well she responded to 'suggestions' by the interviewer and ended up agreeing to everything he said -- it's like the adults pieced together her story and then told it to her so that it formed part of her self-perception (e.g. "You were abused so you are now messed up and bad")
I know there must be some elements of truth that started off the whole thing, but maybe there is nothing especially 'evil' about her -- only the fact that she is intelligent and articulate enough to be coherent in the interview. And to be honest I'm a bit suspicious of the seemingly miraculous cure at the end.
I still don't think she killed the baby birds, since she doesn't remember anything about it. If anything, it could have been an accident and she was too little to understand what was going on. I believe she took the knives and I believe she had every intention of hurting her brother. My friend was repeatedly beaten up by her older sister when she was young, and she would say things like how she wished her sister was dead, and that was from a relatively 'normal' family. I understand why Beth would feel this rage to hurt the world, but I don't think she's close to being a cold-blooded murderer.
However, I totally agree she needed therapy. If she was 6 when they shot the documentary in 1989 that makes her around my age now. Wiki says she is currently doing well and is working as a nurse. If that's the case then I'm happy for her.
Anyway, again this is pure speculation based on nothing