My theory how Tony becomes Doom
When the Avengers went back to 2012, Loki escaped with the Tesseract. The Russo brothers confirmed that timeline was never fixed. That branch is still out there. So what if in that timeline, things played out differently? Tony pushed harder for the "suit of armor around the world" and created Ultron. But this time, Ultron won. He dropped Sokovia, millions died, and the Avengers lost. Cap, Thor, Nat, Clint, Bruce — all dead. Tony was the only survivor. Wanda's vision came true. Tony was forced to live with the guilt, watching his creation destroy everything he loved.
He couldn't live in a universe where he failed everyone. So he decided he'd create a new one. A universe where none of this ever happened. That's when he stopped being Tony Stark and became Doctor Doom.