"My personal vendetta, as you so eloquently put it, is a completely unnecessary safety risk. Harry would never agree to such a thing. Greyback isn't an Exit candidate, after all, and that's the only thing Harry and Ron care about at the moment. If he happened to fall into their hands in combat, they would interrogate him instead of killing him outright, yes, but they would never actively search for him. He currently has too little to offer, so there is no justification for endangering our fighters just to capture him."