I just love murderous!Eggsy. There's something delightfully ironic about how he can't shoot the dog, but has no problem killing Chester King and a bunch of other people. Because the dog is innocent, while those who allied themselves with Valentine are anything but.

Also, the tranquil fury and sheer lethality when he leans forward and watches Arthur die is just amazing.

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Eggsy feels no hesitation, no remorse, nothing but a cold frozen fury when he leans back in his chair (Galahad's chair, Harry's chair) and watches, smiling faintly, as realisation breaks across Arthur's face. He picks up the brandy glass again, sees it turn between his fingers, hears himself talk about the irony of Kingsman's leader falling for a simple, common trick – and it's like he's an observer in his own body. It feels as though he's listening to someone else's voice, seeing through someone else's eyes like how he saw through Harry's glasses…

I'd rather be with Harry.

The ice shatters, and once again grief is burning through his veins. He surges forward, back into himself, with hands clenched together hard enough to bruise and the grim parody of a smile falling from his lips. Eggsy almost wants to bare his teeth and snarl at the man who had allied himself with Harry's murderer, almost wants to let the traitor see the true face of this creature of vengeance he has surely become. But he doesn't do any of these things, because Harry wants – wanted – him to be a gentleman and he wants to live up to that, even when Harry's gone and he's still learning. So instead he fixes his gaze upon what is about to become his first kill, and suddenly thinks of how he couldn't shoot the dog.

I knew you couldn't do it, Arthur said.

"I had that down already," Eggsy says.

He finds himself oddly unsurprised when Chester King curses him in tones even coarser than his own (and oh, how the mighty have fallen), and does not flinch when the man drops dead before him, head thudding against the table as the poison completes its course.

I've had a lot of fun with this, he remembers Harry saying.

Eggsy can definitely see the appeal.

A life for a life, he thinks as he picks up the same pen that nearly cost him his life, and stabs (with just enough force, because he's still trying to be a gentleman, still trying to learn) at the small, neat scar in Arthur's neck.

But Harry's life is – was – worth so much more.

He barely notices the blood staining his fingertips as he pries open the incision and extracts the implant. He stares at it with mingled anger and disgust, before being distracted by Arthur's phone screen flashing purple with the V-Day notification – and it is at that moment that Eggsy knows, just knows without having to think any further, what he needs to do.

In Harry's honour, Arthur told him only moments ago, I am inviting you to be part of a new world.

Eggsy has never heard anything more ridiculous in his life. The only world he wants to be part of is one with Harry in it, but Valentine has taken that away from him.

He'll just has to settle for second-best, then, won't he? He knows nothing will ever be worth the price of Harry's life, but the least Eggsy can do is to save the world and avenge his death.

Chester King was the first man Eggsy has ever killed, but he has no doubt that there will be much more blood on his hands before the day is over, and that Richmond Valentine will be the last.

The King is dead, long live the King.

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I feel like the dog test is, at least partially, meant to test if a candidate capable of having the intention to kill. (It's probably also intended to be a lesson that because the candidate has made his/her choice to pull the trigger, he/she has to live with that decision.) The thing with Eggsy is that he already possesses the kind of mindset necessary for killing, just that he hasn't seen the need to use it until now. My headcanon is that he made the resolution to murder Dean if the man ever went too far against his mother and sister, which is why making the choice to kill someone doesn't faze him anymore.