We are all capable of making wrong choices, that's what free will is all about, nothing automatic about it. Even saints are capable of screwing up once in a while. However which is a mere mistake and which is a clear deliberate choice? In other words emotion of the moment type mistake verses that of a calculated cold blooded monster? When Harry wanted to kill Rudolph, he had just witnessed him killing with a gun his beloved friend and lover, Murphy. The pain and rage he felt in that moment doesn't make Harry a monster or a protomonster, it makes Harry very human.
All humans
are part monster. Rage is one of the things that can release that monster from its cage. That's one of the reasons the Council is so hard-ass about the First Law.
Harry was in the grip of rage that had overridden his conscience and his rational mind. He was, or was close to being, an animal in that moment. A beast. An angry predator. He wasn't seeing Rudolph as he was, he wasn't seeing anything as it really was. If he had given in to it, it would have been a first step down a dangerous road.
What is more consider how much control over the Winter Mantle Harry really does have verses if he didn't. In Cold Days it was all Harry could do to keep from raping any female that moved or not fly into a rage at any slight because of the influence of the Mantle. He has learned to keep the Mantle under control with all that exercise and whatever mental discipline's he has developed for himself. The fact that Harry has worked so hard to keep the inclinations of the Mantle under control proves he isn't a monster or a protomonster. If Harry had really cut loose at the moment of Murphy's death, being a strong trained magical talent turbocharged with the Mantle of Winter, even two Holy Knights with Holy Swords would have had difficulty stopping him,
On the contrary, it would have made it
easier for the Knights. If Harry had really given in entirely to the monster, then the Knights would have been free to act against him without holding back.
Remember what happened when Harry tried to strike aside
Fidelacchius. As he himself tells it, it was something like 'pain beyond pain'. His Mantle instantlyu collapsed, his power fled, he was just Harry Dresden, ordinary human being. If he had tried to use his own personal magic in that moment against Butters, I'm pretty sure
Fidelacchius would have taken that away, too.
Even after the Mantle returned and eased the pain of his other wounds and began to heal them, the burn from
Fidelachius kept hurting, it was a reminder of what had almost happened.
Sanya would have to have used his AK-47 [sorry can't spell the Russian version] on him, and I doubt short of a head shot, that would have stopped him.
In that state, the Swords would be useful against Harry, and they would absolutely stop him. So would a point blank headshot from a rifle, for that matter. Even as Winter Knight, Harry is still mortal.
No, Butters and Sanya were able to talk sense into Harry, calm him down, because Harry's human will prevailed, not the mantle of Winter.
Butters and Sanya were able to reach Harry
after Fidelacchius erased the Mantle and left Harry just Harry again, and after the pain of touching it shocked him out of his 'clarity', as he was thinking of it. He wasn't even listening to them before that.
Before that, Harry was so lost in his rage that he thought the Knights were behaving wrongly by interfering, he was in that
precise mental state the White Council worries about with the First Law: he was ready to kill and thought it was
right. He might very well have used magic to kill Rudolph in that moment.
Remember Harry's mental state a moment later after Fidelacchius freed him from the Mantle. He was
horrified, he suddenly perceived that Rudolph was himself horrified and guilt-stricken, that he had been fighting his friends, the best men he knows. That he had been ready to become a murderer himself, for what was fundamentally a selfish reason (it's not as if murdering Rudolph would restore Karrin, after all).
Sanya and Butters weren't saving Rudolph, they were saving
Harry. If they hadn't been there, there's a good chance that Harry would have broken the First Law, straight up, in that moment...and then it wouldn't just be expulsion from the Council he was dealing with.