Ok, this is an interesting question, not sure where to start ... but I'll give it a shot. And I'm just gonna be completely honest, not meant to offend anyone.
Anyway, as to why women "slut-bash", I have 2 thoughts. The first one is evolutionary-wise, or whatever you want to call it, women are constantly in competition with one another for men, the attention of men, etc. So maybe when they see another woman they perceive as a "threat" or "competition", they want to bring her down immediately ... insult her, bash her reputation, etc. There is probably an element of jealousy in there also, which always brings out the green-eyed monster.
My 2nd thought is purely my own opinion. In all honesty, it just annoys me when I see another woman blatantly acting/dressing 'slutty' or using her boobs only instead of her brain/personality. It really lowers my opinion of her. I understand, of course, that women are, to some degree, going to utilize 'their assets', but some just really go overboard. It just makes me honestly laugh at her, and since she's putting herself out there like a slut, then she shouldn't be surprised if women call her a slut. To me personally, it's just annoying, and I always wonder why they want to present themselves to the world in that manner. Have some self-respect.