Baymax remembered the angry look on Hiro's face as the man behind the mask told him that it was Tadashi's own fault he died in the fire. Though he was meant to help, Baymax felt a rise of anger wash over him, and he wanted to wring the man's neck. Fortunately, his chip stopped him from doing so.
Then, Hiro told him to destroy.
He tried to reason with the boy, because he knew how he was feeling. He was angry, sad, and confused. He told Hiro that he could not harm a human being, because his chip Tadashi had made was stopping him.
Hiro took the chip out.
Every single protocol stopping him disappeared. His emotions vanished, except for one.
Anger.
Oh, how wonderful it felt to destroy at that moment. He said he couldn't remember what happened, but he lied. He remembered the fear on his face, the fear coming off the man who killed his creator. He remembered beating up his friends, their terror astounding, with only one thought on his mind. Destroy, annihilate, kill.
What he remembered most was how he looked. He saw his appearance in the man's eyes. He looked the same as before, but more intimidating, dangerous. With cold, red eyes. He was thankful for the girl who gave him back his chip, though Hiro yelled at her later. He didn't want to kill others, no matter how evil. He was meant to heal and help, not kill and destroy. So when Hiro tried to take out his chip again, he refused to let him. He let Hiro scream at him and try to take it out by force, but he wouldn't be that thing again.
He wouldn't become a monster.
Hiro, being the good-natured boy he was, finally realized his errors and saved the day, even at the cost off Baymax's body. Of course, he was rebuilt, since Baymax left him the chip that held his personalities, but not his evil. That way, he literally couldn't be that again. Baymax flew along the surface of the ocean, with Hiro on his back, just skimming the water with his robot suit. He stared down at his reflection, and almost flinched at it. He would never tell Hiro, or anyone else for that matter, but he didn't like his suit anymore.
He absolutely hated the color red.