Bethune is accused of waving a gun around and shooting 13-year-old Marquis "Little Brother" Perez in inside 186 Howard Avenue.
Perez was rushed to Kings County Hospital, where he was in critical condition and on life-support equipment.
Police say charges will be upgraded against Bethune if Perez dies.
The incident happened Thursday afternoon, with five youngsters playing in the front room of the apartment. An adult was home, but was in a back room. Then, police say Bethune began showing off a handgun.
Authorities say there was no argument and no intent, but the handgun accidentally went off and hit the young boy in the temple.
Police say Perez was not playing the gun, as was initially suspected. Now, there is inconsolable grief in Bedford-Stuyvesant.
"There was a couple of kids up in the house, they were playing around, and one just happened to have a gun on them," community activist Anthony Herbert said. "The gun went off and shot the other one in the temple. Just an unfortunate 12-year-old."
The tragic shooting underscores two troubling problems in this neighborhood: too many guns and too much idle time.
"It's a shame," Herbert said. "I do support the mayor and the efforts that he's doing with gun control. But we have to do more. I think what's happened is we just have a bunch of kids who are idle. And they just need an opportunity to do things. Unfortunately, they're cooped up in the house. We need more recreation centers and stuff like that. I think that would give them an opportunity to survive a little longer."
Police say Bethune ran when he realized what he had done. He is in custody, and says he did not know the gun was loaded.
