BertrandRussell firstly, I have said I can see how they have a history, how the word developed into a 'offensive word' from it's original meaning. Why? I personally think it's the taboo of the era it emerged from. The fact they, in medieval times couldn't even bring themselves to describe a woman's parts but through slag doesn't suggest a 'hatred of women', merely embarrassment of their mention. It didn't start as 'the most offensive word' - it probably became a dare of the time to say it. Possibly a joke term, especially in certain circles (sailor talk as it were).
Why don't I believe it's purely historically derogatory to women? Well as many here have said, there is a huge history of demeaning behaviour and language towards women. So logically speaking, shouldn't be using 'cunt' as an insult be less offensive than it actually is? History shows us that society has had little care to use derogatory terms both about and towards women. It seems more the case that 'cunt' has something 'more' about it than just another way to undermine women as a whole.
What precisely makes it such a vulgar word as a swear is a rather complicated matter. It's original form was bore likely out of embarrassment (using slang rather than speak of a woman's genitalia in any way), the taboo making it a 'dare' to say, an element of misogyny, an element of semantic broadening as it evolved over time into an insult and the change of language and the culture it was used in. All language and words change over time, any word can change its meaning completely to the original, even if the original still exists. So whilst I'm willing to accept there may have been an original misogynistic origins to the word, it was such a long time ago that the secondary meaning has long become a stand-alone version.
Ultimately, if there was a correlation between the swear being a purely misogynistic term, there would be far more historical evidence of the fact. Rather, it seems it was 'rough language' that in recent years became unacceptable in developing modern society and when put next to the original meaning became a very big 'no no'. So in short, the original meaning is possibly why it's such a 'bad word', but more of the taboo and embarrassment of it being an old slag word for vagina, rather than derogatory towards women.