Ehhh, the swords aren't made to intercept evil acts though. They facilitate choice.
Which fits with what I am saying. The burn presented Harry with a choice, the smell of sulfur and brimstone, the consequences of that choice, ice water to the face, "wake up, there are consequences, is this what you really want to do?" Harry came back to himself, the answer was, "no, this isn't what he wanted to do."
Harry's act may have been evil, using defensive magic to kill might have been a twisting of creation itself. But Harry's human, he's allowed to error.
Correction, it would have been evil had he carried it out, but the burn brought Harry back and he chose to let Rudolph go. Kind of a play on a Holy Knight giving a Denarian a choice, surrender the coin and seek redemption or continue his or her evil ways. The burn made Harry pause long enough to make a choice.. And yes, I agree Harry is human, humans error, and he had been under tremendous pressure and snapped.. Forgivable, even understandable, but if he had killed Rudolph
that bell cannot be unrung and Harry would have had to live the those consequences.