"What's happening?"
The words cut Tony to his core, and he fought down the rise of nausea as he turned to Steve, just in time to watch the bigger man fall to one knee. Blue eyes locked onto brown as the Captain wavered, his strength seeping away.
"Tony, what's going on? What did you do to me?"
"It's a new drug," came the dead reply, "a mutated form of the serum that Dr. Erskine injected into you, but where Erskine's formula made you stronger, this does the opposite."
Steve's face remained stoic for a moment, then his eyes widened in horror as he realized the depth of Tony's words.
"You're unmaking me?" he asked, struggling to stay upright.
"No, Cap. I'm killing you."
Five words. Five words was all Tony needed to take the wind out of Captain America's sails. Steve's fair skin was already beaded with sweat, his muscles failing one by one, and he slowly began to sink to the floor, unable to hold himself up.
"Why?" the blond man croaked, the break in his voice a mixture of pain and his impending death. "Why are you doing this?"
"We need the SRA," Tony said simply. "It used to be different, back when it was just the four of us, but now everyone and their brother has some kind of super power. Everywhere we turn, there are mutants, enhanced super soldiers, assassins, spies. We need to keep them in check. And to do that, we need a list."
"It's wrong," Steve wheezed, trying desperately not to collapse. "It's wrong to make people reveal something that private."
"It's wrong not to." Tony's eyes were steel but his heart was crumbling, and it took an indomitable amount of willpower to keep the sadness from his voice. "We swore we'd protect the people, Cap. The ones who couldn't protect themselves. This furthers that goal."
"No. You're compromising everything we stand for."
Tony shook his head, tears welling.
"The logic is sound," he finally said. "I tried to make you understand, Steve, but you wouldn't listen. You never listened. Not when it was important." Steve opened his mouth to say something but instead he groaned and fell to the floor, face red and muscles constricted in pain. Tony moved to him and knelt, one hand gently pushing back blond hair, but Steve only glared up at him and tried to pull away.
"You were my friend," he ground out, teeth bared in agony.
"But first I was Iron Man," Tony said. He felt one rogue tear leak out and burn a hot trail down his cheek. "And that's where my duty lies. I'm sorry, Steve." Rogers fought to say more but as quickly as the pain had come, it was gone, replaced instead by a bone-deep weariness. Suddenly, he had to fight to keep his eyes open, and he knew the poison was working quickly.
He only had seconds to spare. He only had seconds to say the words that would stick,
"I'll never forgive you," he vowed as his eyes closed. Tony waited a beat, watching the Captain's chest in suspense, until finally it stopped moving. For a moment, he felt numb, unable to believe what he had just done, and then the tears flowed freely and he buried his face in Steve's chest as he cried for their lost friendship and the Captain's lost war.
"I know," he whispered to the star-spangled corpse. "I know, I know, I know."