Butcher himself has chimed in and said that intentions matter little in terms of "corruption", especially in the face of results. I believe the analogy he used was shooting someone in an attempt to wound, but killing them anyway. You may not have intended for them to die, but you still fired a potentially lethal weapon at them. It doesn't matter if you didn't mean or want to kill them, what matters is that you did. Now, you may not agree with that philosophy (I know I have a few issues with it), but it is what Butcher has defined as causing the black magic corruption.