In the attack at the small store someone summoned the Walker.  The Walker was the test, but test was to see if Harry could fight back from the psychic attack. He didn't defeat the Walker he exploded the fuel tank. Butcher supports this in Peace talks during the fight in the parking garage.

Yes and no, the exploding gas tank did end HWWB's attack, but up to that point young sixteen year old scared shitless Harry Dresden wasn't exactly buckling under the Outsider psychic attack either.  Impressive since young Harry had no clue as to what or why he was being attacked.  He had no defense since he didn't know at the time that there was such a thing as a psychic attack, or how to defend himself from it.  He kept up his adolescent smart ass fall back defense act that we've all become familiar with, and it more or less worked.  We don't know how long it would have worked because HWWB decided to murder the gas station attendant, which horrified and piss young Harry off and he ignited the gas pump.   So while perhaps not definitive, whoever sent the Walker did get quite a bit of data on young star born Harry Dresden. 

It's entirely possible that *every* member of the White Council has their own personal agenda for Harry... because they're very-senior wizards (aka arrogant and self-confident to a very-nearly pathological degree).
 

Yes it is possible, I do think as a group they had an agenda for Harry, but their mistake was thinking
they could control him. 

Snapping implies a loss of control.  In terms of character development why did Butcher have Harry lose his shit? What did the passage accomplish?  What was it meant to do? I understand the emotional shorthand that Butcher is using, at least in part.

  Butcher is saying first and foremost, Harry is a human being.  His reaction given his love for Murphy and the stress of being on a war footing is normal!  Audie Murphy spoke of the same thing happening to him in a battle, his best friend was killed in front of him, something snapped and he took out a whole hill of machine gun nest all by himself. That psychologist says it is built into the human brain, it is a normal reaction though not all are perhaps as extreme as Harry's was, but there are those that are.

Not anyone.

Yeah, anyone, if you have never had that kind of reaction you don't understand.  When our son killed himself, my husband tried to stop me from going to his room.  I literately ran over him in the hallway to get to his room.  Nothing was going to stop me.  Apparently two trained Holy Warriors couldn't stop Harry from going after Rudolph.  Oh they could have shot him, I suppose, but other than that Sword burn, nothing else would have brought him back to his senses.

If Michael had stood in his way?
Mouse?  You think he would have attacked Mouse?

No, Michael wouldn't have been able to stop him.  Mouse perhaps with sheer physical animal strength, taking him down and sitting on him. Or maybe not even him because the Winter Knight Mantle would have taken over.  We are talking about the same combination of hormones that take over that allows an 80 pound kid to lift a car that has fallen on his father.

Sanya is a very-iffy "maybe" to me:  Harry knows Sanya as one of the "good guys," always an ally, always trustworthy.  But also, always a fighter; so I can kinda-sorta see Harry fighting Sanya to get what he wants so badly (to kill Rudy).

Rationally maybe, and you'd be right, except when a person "snaps" they are no longer thinking rationally. 
As Dr Fields says

“Why We Snap” outlines nine, but some of the most common ones are a life or death threat, threat to a loved one, threat to your home, or threat to your tribe. “Our brain is wired to constantly be on the lookout for threats,” Fields says. “In response to sudden danger, we react automatically; you can't think about

Butters, I think, is off-limits to a Harry not "under the influence."  Butters has just barely emerged from the "helpless nerd who needs protecting" stage of things.  I can see rage-Harry pushing Butters aside, shoving even.  But not an outright attack with deadly force; again:  unless "under the influence."
 

Which proves the point actually, a rationally thinking Harry would never attack Butters.  Heck even in Skin Game when Butters was attacking him, Harry did his best not to hurt him.

Butcher is showing you the monster.  The other Harry. the Harry that is arrogant, full of rage, and absolutely cock sure of himself.  Who is certain that what he is doing is right, and the way it's supposed to be. 

No, Butcher is showing us a human being.  You and I under the various circumstances are capable of snapping just like he was.  Oh you can claim you wouldn't but apparently it isn't a voluntary reaction according to Dr Fields, it is wired into a part of our brains. What you are saying might apply to Harry in other ways, but had nothing to do with his reaction to Murphy being shot and dying in his arms in that moment.

My contention is that now you know.  Harry was programmed to react in this way.

No more that you are or I am... He maybe programmed but his reaction to Murphy's death had nothing to do with that.

I feel sorry for Harry because as a character he's been abused by people close to him. He was programmed to be a psychopath.  He's trying to be something more.  I wish I was going to see how it all turns out.

No, he isn't a psychopath..

Psychopathy is a maladaptive personality disorder character- ized by such traits as a lack of remorse, manipulativeness, egocentricity, superficial charm, and shallow affect (Cleckley, 1941; Hare, 1991).

Harry was totally remorseful, and he feels empathy for those that suffered and died, he is no psychopath.

His utility to the narrative is at an end. There is no point to internal White Council politics now they have thrown Harry out.

My question is did he ever have any utility?  Oh we readers did a lot of speculation about him, but as a character he was almost nil to begin with.  Of the Senior Council, the members that have had any significant interaction with Harry are, Eb, the Merlin, Rashid, and Listens to Wind.  Martha Liberty got a few lines, but between Summer Knight and Battle Ground she may well have never existed, the same goes for Ancient Mai

I'd put the White Court up as human, or at any rate they started out as human, and someone made a deal, with a demon called the Hunger.  It is a parasitic relationship, or symbiosis

Symbiosis is a relationship between two organisms: it can be mutualistic (both benefit), commensal (one benefits), or parasitic.

The Red Court seems to have been different, in that it was more of an infection..  But an infection with plenty of benefits, it gains nothing by remaining human, so when fully turned they no long resemble the humans they once were unless they require the illusion of it to gain more victims.

Never say never.  But Harry has had rage issues since Storm Front. Where he proclaims...Seems pretty rage like to me.

Issues?  Um the Shadowman was getting ready to explode his heart remotely.. He proved he could do it, Harry saw the results.. So hell yeah, he'd want to kill him before he was killed.  The issue isn't that he contemplated doing it, the issue is why didn't he?  People in a rage don't stop to think, they just act.. That scene doesn't at all demonstrate "rage issues" on Harry's part.

The brief bit of Harry we see in the fragment of Dread Beast Mister is very different to the Harry we see in Christmas Eve, as is the Harry at the end of that story.

I think in Dread Beast Mister, we are going to see a Harry wallowing in his misery in the Castle. Whatever Toot does it pushes Harry towards being the more functional Harry we see at the start of Christmas Eve, willing at least to put on a facade

We see Harry grieving, not wallowing, but grieving, for crap sake he still is wearing a cast and bandages, its been weeks, not months or years!  It isn't just Murphy that he lost, but for the foreseeable future his brother Thomas as well.  I'm not sure how you expect Harry to act, but it looks quite normal to me for someone who has lost so much.  He is attempting to get his emotions together, that is why he is meditating, that isn't wallowing.  Yes, he is doing better by Christmas Eve, a little time has past, at least his physical injuries have healed and he can do more to cope with his grief.  I am sure Toot does help him, but grief cannot be turned on and off like a faucet, it is a process, even for a wizard.

You may be right.  I don't think so but I am not Butcher. So it's a personal bias.  On Reddit there has been some talk that he's told the betas that it will all come out in the end.

If we all live long enough to see it... It may all come out, but that doesn't mean it will be satisfactory..