What’s up with women that they can seem nice and then basically on the flip of a coin can be horrible people?

[coldmarch]

I think this happens especially for those women who are very, very, strictly, "territorial", meaning that they have only a list of "allies" and the rest of people are all "enemies". Those who try to keep things divided, who can't see grey areas in the middle. They'll be nice, available, generous and loving with their allies, but the worst passionated hater of anyone else. I think this kind of dualistic territorial behaviour is more prevalent in women and homosexual men (or at least this is my experience).
So when something goes wrong in a relationship or a friendship, they flip very quickly and you become instantly an "enemy", and treated as such. I think this mentality has something to do with a generic very strong insecurity, very low self esteem and fear of abandonment.

Another case is: some people (any, males and females), don't discuss problems straight in the face because can't handle conflicts or don't feel enough assertive to expose a discomfort and would rather passively try to tolerate, while accumulating, then all of a sudden releasing and venting out everything. Not sure this means being "horrible people", though.

You didn't add enough details to analyze your situation so I'm being quite blind and generic in what I'm saying...

[WesBrewer]

Sometimes people are different from what they appear, they change when the situation changes.

[jgibsonian1986]

In addition to what said, bipolar disorder is also a thing.