Here is another great post from LabRat of Atomic Nerds. It is about gender and aggression, and why it is a fallacy that women are inherently less aggressive.
I'll add that, in my experience, the women who study martial arts are more fierce and aggressive than the men. Once they get over the cultural conditioning against violence, their instincts seem to be more aggressive. When you spar a guy who is just learning, you can usually tell that he knows it is just a game. But when a girl starts fighting, you have to be careful. They are less likely to pull their punches, partially because they don't know their own strength but also because the concept of play-fighting isn't part of their instincts.
Of course, after a few years of study in any high-quality martial art, everyone learns how to control their instincts and be calm and peaceful. The effect is similar to the one discussed here.