One was swapped with another. Both had the same intention...to go into the fire. After the flames one was dead. No mater which hamada, "Same story they said." No matter the story it's still always the same, a brother torn apart by a fiery flame.
The death of the sibling was all because of a traitor who would surely be exposed sooner or later.
Though the hamada brother had a plan to catch the treacherous man, it was quite unstable. Turn a robot against the morals of his dead creator.
When the traitor was cornered the mask was lifted causing the plan to now have shifted. Seeing who it was ignited a spark of a thought in the boys head. It would be better if the traitor was dead.
"Baymax destroy!" where the last spoken words of the boy before in the push of a button and the flick of a wrist out flew the robots healthcare chip. It now wasn't the cuddly marshmallow it was intended to be. It now became a killing machine. The boy learned his lesson at the end of the day as his friends put the chip back in its place.
His whole plan was all for his dead brother. It was to get revenge for the harm inflicted by another
The theories will run rampant and wild, that maybe a brother survived the fire. Though we don't know if this is true Tadashi lived on in spirit watching as Hiro grew.