"Don't hurt my dog or I'll kill ya." Eggsy called across the barracks towards Charlie and Rufus who were making comments about just putting the dog out of its misery.
Just because JB wasn't as big as any of their dogs or as physically able at the moment, didn't mean that his dog wouldn't become better than theirs. So he was a pug, and not a bulldog like Eggsy had thought initially, that didn't mean he wasn't a good dog. JB just needed a bit more training, things would all work out in the end. Unlike their dogs though, JB was already a good mate to him, they had bonded. The only other person in the barracks that seemed to have an actual connection to their dog was Roxy.
Merlin had said that getting a puppy was a teamwork thing, and Eggsy could see that; he and JB were definitely a team. The little pup was coming to mean a lot to him, he accepted Eggsy as he was; which wasn't something many did, so he would do the same. He accepted JB the way he was and would protect him from anyone who tried to harm him.
As Eggsy left Kingsman for the last time he cursed Arthur, Merlin and Harry; all that talk about teamwork only to expect him to kill his dog. It was cruel, JB was his family just as much as Daisy and his mom were, he wouldn't do that. He took the taxi parked out front, he may as well get something from Kingsman after all, something of worth; they didn't seem to think his dog was worth it after all. Finding out that the gun was filled with blanks the first thing he thought was that Arthur should be shot, psychological torture was worse than physical torture in his book. If he had gone ahead and shot JB he would have survived of course, but the bond between them would no doubt be broken. The one person that is supposed to care for you betraying you, nope Eggsy was glad he hadn't done it; even if it meant that he would never become a Kingsman.
He had protected his dog, just like he would protect his family, if Arthur didn't like that, well Eggsy didn't care. Arthur had some pretty screwing ideas about protection and sacrifice that Eggsy wasn't going to get on board with. When he switched their glasses and then watched Arthur realise what Eggsy had done he had two thoughts; that's for Harry and that's for JB.