You should be more mad about the fact that she's a cruel and sadistic villain, not the fact that she's into women. If she was straight, would her make you a '''straightphobic'''?
Another example, if instead of her, there was a black character who's just as cruel and sadistic as her, would you say that the character is making you racist?
Food for thought.
If you say no to both my questions, then It means you have no problem when a villain is straight or black, rather, you have a problem with their cruel actions which of course is understandable, but you seem to have a problem when It's a homosexual. You might say that you are not homophobic right now, but I feel you have this inside feeling of homophobia.
As I've said, hate the villain for being cruel, for being sadistic, for being annoying, whatever. But hating a villain because of their homosexuality is not it.