He didn't even realize, in the shop.
Eggsy had been excited, in a way, when he and Harry were walking to the tailors' shop and he realized that the tug he had felt all his life - faint and distant for so many years, until suddenly it started moving in so many different directions - was getting stronger, nearer. Would they be a customer, someone rich and powerful that Kingsman clothed, someone who could never fathom the life he had lived? Would they be a Kingsman, one more person for whom he had to pass?
Of course he was distracted by all the amazing shit in fitting room 3, and then the whole confrontation with Valentine, so he didn't notice her. Didn't look his soulmate right in the eyes, didn't recognize her. He barely even noticed the tug growing fainter, later, when everything was falling to shit.
Now here he was, watching her jump down in a shower of glass with a gun pointed at his face. For one second - just one fleeting moment - their eyes met.
He saw, mirrored, felt the shock in her eyes that this would be how they would find their soulmate. Shock, and brief horror - and then the moment passed, and all that was left was furious determination.
How fitting that the only way this could end was one of them killing the other, he thought, as he dodged her bullets and blades, danced a deadly battle with his sharp soulmate, and how fitting that she died from a blade on his foot.
He took the prosthetic off one of her legs while she lay there twitching, caressing the stump as though in apology. He stepped back and hurled it at Valentine, turning back to Gazelle once he saw Valentine fall on the blade.
In his ear, Merlin was congratulating him and Roxy on a job well done. Eggsy ignored him, crouching over Gazelle. "Harry would be proud - Eggsy? What are you doing now?" Merlin was asking in his ear, confused.
Gazelle looked resigned, now that she was too weak and twitching still, but she had just enough motor control left to lift her left hand. Eggsy took it in his right, lacing his fingers in hers, and kissed her fingertips as she finally smiled at him, before her face went lax and her laboured breathing stopped.
Eggsy sighed as he straightened up and went to check Valentine's body and confirming his death.
When he got back to the plane, Merlin greeted him with "What was that back there?"
"She was my soulmate, Merlin," Eggsy murmured, not meeting his eyes as Merlin's expression softened and he patted his shoulder in apology. "Gazelle was my soulmate. But only one of us was going to walk away from there, and if I died so would too many people, so I killed my soulmate."
"I'm sorry, lad," said Merlin. The lad couldn't shoot a dog, wouldn't run over a fox, but he willingly killed his own soulmate to save the world. It'd be a cold comfort, he knew, watching Eggsy barely holding himself together, but damn anyone who insisted after this that he couldn't be a Kingsman.