Oh, I'm sure she was trying to at least annoy the Generous Ones. She didn't just see it as an 'easy' way to dispose of the nemesis. As to whether John created the Nemesis from bits and pieces like Frankenstein's monster, or summoned it from some other realm and placed a geas on it to do his bidding, I don't think is ever made clear, is it? Not that it really makes a difference at this point.