Mako had only intended to surprise Bolin. Had only wanted to do something nice for his little brother while he had been out with their parents. Bolin had just started school - and had trouble fitting in with the rest of his class. Mako thought - Mako felt that getting Bolin a small gift - that having their parents take him out so he could buy that small gift with the pocket change he had been saving - that it would help Bolin feel less anxious. But he wanted it to be a surprise, too. Bolin couldn't come along!
Of course his parents wanted to come with him. He was only eight, after all. Republic City was huge. A dangerous place.
So when the Agni Kais surrounded them, Mako tried. His mother just told him to run - told him to be safe, to hide in the small alleyway, where she had bended the ground to cover him - to hide him from their attacker.
He remembered her telling him to stay there until she or his father came back. Not to move unless he absolutely had to.
He wished now that he had listened. That he hadn't stepped out of the hiding spot his mother had made for him. That he hadn't watched as his mother crumpled to the ground first as lightning struck her, and then his father. He ran - ran back home, found Bolin, and they stayed there for as long as they could.
He was only eight. Bolin was only six.
And now - now they were orphans at the mercy of Republic City's streets. Mako couldn't count the number of times he held Bolin as he begged for their parents to come back. Couldn't believe how he came to rely on those same streets to take care of them - as long as they stayed clear of the Agni Kais.
But most of all, he couldn't fathom how Bolin had never come to hate him. How Bolin always smiled, just for him, and never blamed him for their parents' death.
Mako guessed he blamed himself plenty for the both of them.