Harlot and many other words can mean the same thing, but do not come across as damaging as the word slut. It's one of those short, sharp words that can be used in language much like a knife, it can be hissed or spat out quickly in an attempt to harm or hurt.
In other ways there have been women who have called themselves slut to me, not being degrading of themselves, but in description of how they themselves may 'act out' on occasion.
Personally, whore is a word I've never been comfortable with. Don't know why, just that it is. Maybe the sound or it's meaning of pig is offensive to me.