I don’t hear Morraine call her a “beast”…Morraine says she’s an “ancient being that rules the night…the original power”. I’m not sure what BF’s intentions are, but she did tell Bae that there was “good magic and bad magic” and she was on the right side..which is probably “good” magic.
“Good” can be a matter of perspective though. I think slurpeez’s summation of her motives in the post above mine is spot on.
No matter the situation, BF will always deem herself to be on the “right” side, but I don’t think that means she only uses “good” magic to ensure things play out the way they’re meant to. If she is indeed the original power, then she must be made of both light and dark.