No need to feel silly! Despite a few counter examples, the basic premise of your post is valid: sexiness is often used as a short-hand for female aggression/evil, even in works that are otherwise well written. I see it with male characters, too, but so much more with female characters. It's rampant in a lot of the stuff I like to read or watch, including science fiction, fantasy, horror and comics. Each work needs to be judged on its own as to whether or not it uses that trope successfully, but the overall trend is disturbing because it suggests a mindset that I don't like: the idea that female sexuality is threatening.