Disclaimer: I don't own Star Wars and I'm not making any profit off this story...except working through emotions stirred by the movie. Enjoy!
FN-2187 never had a father...at least, not one he remembered. He was raised as a weapon and weapons don't need fathers or mothers. Or...whatever it is that Rey was to him. He wanted it all, though. He hadn't known how much until just a few minutes ago, when his enemy and his ally revealed themselves to be father and son. Then the son had killed the father; now the son stalked toward Finn through the snow. Rey lay still and unconscious. Her blaster was far away and the only weapon Finn could reach was the one he'd already lost a fight with.
"There is no fight against the First Order," he had said, "at least not one we can win." How right he'd been, Finn thought. He hadn't expected not to care. It was a simple thing, to face Kylo Ren and turn on the lightsaber. Much simpler than committing high treason. His world was illuminated by the hero's weapon, and by that light, Finn saw the truth. They stood there, not as Master of the Knights of Ren and renegade Stormtrooper, but as two men called by names they were not born to. Two men who had chosen their sides in spite of all the influence of their upbringing. Ben Solo had been born to heroes and FN-2187 had been raised to be a weapon...and yet here they stood.
"That lightsaber, it belongs to me," Kylo Ren said and Finn felt something piece itself together inside him.
"Come get it," he replied, and then he charged. For Han Solo, who was both smuggler and war hero. For Poe Dameron and the Resistance he served. For Luke Skywalker, the hero he had been told to hate. For Rey, crumbled in the snow behind him...Rey, who still looked at him as no one ever had. But also for himself; for the family he never knew, for the name and the life that had been stolen from him. He was Finn now and he couldn't bring himself to regret that...even as he wondered what might have been if not for the First Order. If Kylo Ren and the First Order wanted to steal anything else from him-this lightsaber included-they would have to pry it from his cold, dead hands. He was done trying to get out of their way.
The fight lasted longer than he expected, but it end it did. As his world dissolved into red-tinted agony, Finn wondered what Maz Kanata would see in his eyes now. /div