I just had a thought: Why is it, it comic books and TV shows, whenever a good guy manipulates the weakness of a bad guy to his own advantage, it's a clever strategy, but whenever a bad guy does the same, it's a low blow?

I just realized that, and this quote came to mind. It's from 8Bit Theatre:

Garland: And what kind of tactics do you useually employ?
Fighter: Oh, we usually murder our way to the top and claim victory astride a pile of mangled bodies. But we're herose so it's okay when we do it.

So, basically the moral here is "Be a good guy, because then you can get away with anything."