The weight of one's soul is not immutable. Various factors can change it, taking away or adding to. Emiya Shirou chose one to become a hero. Tanizaki Shirou would chose another.

Fate: Creation

Gilgamesh looked at the brat before him. A thought occurred. "Child, what do you wish to become in the future?" Tokomi had explained the purpose of magus-kind and he found it laudable. Knowledge is always a good thing. When he remade his world how he wished, he would keep researchers around. Shirou pondered for a minute. He had just watched an episode of Kamen Rider so... "I want to be a hero!" Gilgamesh smiled. It was good that people still wished to emulate his magnificence. Suddenly, he was struck by inspiration. Before him, all others were mongrels. Could he make this one a hero? Of course, he is the King of Heroes. "Child, I have two gifts for you." Gilgamesh declared. Shirou was floored. He saw a show like this yesterday! A mysterious stranger bearing a gift to change someone's life! Gilgamesh thought for a moment. Irony had always appealed to him so, 'Bring me a Japanese weapon. Something powerful.' With that thought a sickle attached to a chain with a spike on the end came out. With a thought, it dematerialized into Shirou's body. His eyes lit up. "What was that mister!?" Gilgamesh smiled. Curiosity is a good trait. It discourages stagnation. Still... "You will find out if you start growing the right way." There. That should start the ball rolling. "Now then, drink this" Gilgamesh pulled a phial of a golden liquid from the gate. "I will check your progress after I have reclaimed my grail." Having said his piece, he departed, and Shirou drank. A+ ranked Charisma had its uses. What was one treasure to Gilgamesh? If he decided to use one than he would never regret it. And then he left, for the final battle was nigh.

And then there was fire.

And all burned within.

And one Tanizaki Shirou saw it from the very edge of the flames, how they danced in joy as they stole life, how the smoke leaped down their victim's throats, and his childish dreams hardened, and grew. He made a vow at that time, to never let destruction like that happen again. For the sake of others, he would become a Hero.