If Tony got the serum, he'd end up like Red Skull
Dr. Erskine said that with the serum "good becomes great, bad becomes worse." Tony is actually pretty awful. Unless he's directly affected, he doesn't care how his actions (or negligence) affect others. He treats people like they exist for him to ogle, mock, or take advantage of.
Howard Stark was self-aware (and smart) enough not to take the serum himself. Tony thinks Ultron and Edith are good ideas, so let's see what would happen if after Civil War he found Howard's notes on the serum.
***
Tony laughed as he set up the last syringe. The glowing blue serum was difficult to make, but Tony knew this was his destiny. Finding his father's notes hidden in a dusty old book was a sign that TONY was meant to be a super-soldier.
And after the serum made Tony stronger and faster he'd show them ALL. HE'D be the suit of armor around the world. And he'd find Captain Two-Face and all the Betrayers and teach them NEVER to mess with Tony Stark.
His dad had stressed that only the best candidates should get the serum. His notes were full of dire warnings that made Tony laugh. Dear Old Dad should have just admitted he was a pussy too scared to embrace greatness.
The lights in Tony's lab (along with his beard and eyebrows) made his smile look almost demonic as he pushed the button to start the injections and radiation.
Then he started to scream...
***
Vision and Rhodey landed at the end of the deserted street. Buildings reduced to rubble smoked all around them.
Rhodey's faceplate flipped up as he said, "Well, that didn't work."
Vision nodded and said, "He is more formidable than I expected."
Tony--or Iron Master as he was now calling himself--was blasting fighter jets out of the sky, laughing as each one crashed and burned. He shouted, "Just stop already! I'm only defending myself! I'm here to PROTECT YOU! Bring me Captain Reject and his little puppets so I can deal with that star-spangled menace!"
Rhodey wasn't sure if there was anything left of his friend. The serum seemed to have turned Tony's skin into a kind of armor. Tony wore a harness similar to Ivan Vanko's, just an arc reactor hooked to gauntlets, boots, and a helmet. Tony's exposed skin was pale and almost gleaming. He looked eerily like Ultron.
Vision frowned as Tony flung the next jet into the side of the building. It exploded, leaving the pilot no chance to escape. Vision said, "Ground troops are withdrawing."
Rhodey winced. The Army hadn't done any better against Tony than the SWAT team and regular police officers who were now a pile of corpses littering the streets. He said, "We gotta stop him."
Vision looked at Rhodey and said, "We cannot contain him."
Rhodey nodded. Maybe if they'd had the Hulk or Wanda they'd be able to capture Tony. But just him and Vision? No chance--and no way they could wait for Steve's team to return from wherever they were hiding.
Vision blinked and said, "Secretary Ross has authorized lethal force. He says if we don't stop Tony in the next fifteen minutes, 'maybe a nuke will make Stark show some respect'."
Rhodey closed his eyes. Thought about the years he'd known Tony and all the wrong turns he saw Tony take. No wonder Tony had turned into this monster. Rhodey said, "Here's what we're going to do."
***
Rhodey landed next to Tony's corpse and collapsed. His suit had barely survived Tony's last blast.
Tony's head sat smoking a few yards away. Vision had shot the laser that decapitated his creator, but Rhodey was the one who'd given the order. Tony had given them no choice, firing point-blank on Rhodey while calling him "a gimpy, wimpy ex-friend."
Rhodey wondered if Tony's face really froze in a sneer. But instead of reaching over to open Tony's faceplate, he told Vision, "Burn it. All of it. Not even ashes left."
As Vision nodded and began his work, Rhodey leaned back and wished Tony had been a little more humble--or at least a little less stupid.