Innocence
He was innocent; and that's how Mako always wanted it to be. The firebender, after all, had devoted his life, ever since they became orphans, to protecting and taking care of his little brother. Mako just didn't want Bolin to have to grow up too fast, like he'd had to.
So Bolin didn't. He stayed a naïve, trusting, caring person in spite of their rough life on the streets. Maybe that's why people like him so much, Mako thought, watching his brother lounging on the couch, scratching a sleeping Pabu behind the ear. Because he's everything people don't expect from a street kid. Because I was that person for him.
Mako had never really thought about it before, but maybe he hadn't done his brother a favor by shielding him from the harshness of reality. When Bolin was captured by the Equalists, when his bending was almost taken by Amon, when he woke up screaming because of the nightmares...Mako felt guilty about it all.
The firebender had seen the fear in his brothers eyes as the truck sped away from the Triple Threat's Headquarters. He had seen the terror in his face as he was shoved to his knees in front of the mob of Equalist sympathizers. And Mako couldn't help but feel that if he had let Bolin grow up a bit, mature a bit, he would've been fine. Well, maybe not fine, exactly, but he could've handled himself better. He felt that if he hadn't made Bolin live under his rule for so many years, his brother wouldn't have felt like he had to go with Shady Shin to please Mako. He wouldn't have felt like there was a debt to repay.
He could've spared Bolin so much pain if he had just sacrificed a little bit of that innocence.
But he couldn't do it. Bolin was exactly how Mako wanted him to be.
Bolin was himself.
How could he take that away?