Uncontrolled rage was one of the early signs of the Shadow getting into Harry's mind, influencing his actions. We can say "But the Knightmantle..." and we can say "But watching Murphy die..." and neither of those is demonstrably "wrong" at all; but also, what if those are just "excuses" that Jim wants us to accept?
Yes, but his lover and long time friend had just been murdered before his eyes in a moment when his emotions and adrenaline were already running high aided by the Winter Knight's mantle in readiness for battle.. So you could be right, and I would agree if this anger came out of the blue or for the usual reasons that get Harry pissed.. However in this case it wasn't usual at all, also the Holy Knights are also Harry's friends and were trying to stop him from doing something he would regret... No different from when Harry stopped Michael from beating the priest who had kidnapped his daughter to death with a baseball bat.. There was no dark shadow of the Fallen occupying Michael, he just lost it because this guy had harmed his daughter. Harry had lost it because he had just witnessed Murphy being murdered, and wanted to kill her killer.
What if?? Is an interesting game to play, but the example of Harry going a bit berserk after Murphy was senselessly killed and died in his arms doesn't even come close. Even if it turns out to be true as you say, Murphy being murdered and dying in his arms isn't one of those "ah ha" moments that prove that the dark shadow of Lasciel remains and is influencing Harry.
I return to my central observation: all of the Knights in the world were in the field at that moment to stop Harry Dresden; the Knights whose core purpose is to oppose the Denarians.
Except that wasn't why the Knights took the field is it. They took to the field to help fight the invasion of Chicago by the Fomor. In the end they put the hurt on the Titan, and were hurt themselves helping Harry to stop the Fomor. Yes, they tried to stop Harry from taking revenge after Murphy died, but that was about stopping a good man from doing something he'd regret afterwards.. Another moment when Harry went a little too far over the line and the Sword of Faith gave him a nice burn to remind him that he did. Did Harry feel rage? Did he try to kill Butters? Or did Harry feel shame and continue to feel it because he knew he had gone too far? Denarians don't do that, Harry is no Denarian.