Cercei is full of hate towards other women, especially younger, pretty and in any position to engender her position. However I think she always wanted to have the freedom and position she was not allowed to have as a woman. I think that all that struggle against the rules of society and losing again and again made her hate her gender and anyone who wasn't close to her (and that meant Jaime and the kids - them she would forgive anything). It doesn't really make her a better person but at least let one see where her obsessions and paranoia come from.