I have seen a lot of this attitude from places (subreddit, r/niceguys) as well as friends telling me of said experiences. A lesbian friend of mine has also ran into girls who do this.
Why do people do this? They snap when they can't achieve what they want? What's the point? What do they hope to achieve from this?
A shorter example below:
A: You're beautiful and would like to take you out on a drink.
B: Sorry, not interested.
A: Doesn't matter, you're ugly anyway, bitch.
Most Helpful Guy
Probably because their egos get wounded which to women may not seem like a big deal because they are never in that situation (believe me when I say women act the same way when they put themselves out their and get rejected) or it could be that they have been rejected multiple times (again women generally don't have to experience this) and they are just frustrated and this is the only way for them to feel like they have some kind of control over the situation. Not saying its right, but its also not just malice either.
Most Helpful Girl
They are not nice to begin with and are sexually frustrated. If you sleep with them, they will degrade, stalk and destroy you anyway, they have problems, run.