I think the main trouble I'm having with your argument is the idea of it "harming" someone. While that seems to perhaps be true in the most technical sense, I'm not sure it's a sufficient argument for banning insider trading. After all, many things which harm people are not generally considered to be bad. For instance, if someone opens a restaurant Ina town, and runs the restaurant very efficient while offering a high quality product, that could absolutely harm other competing restaurant owners. And yet most people do not condemn the practice of opening a restaurant and operating it well.