Nearly half a century after Bian Zhongyun was beaten, kicked, tormented and left to die, bloody and alone, at the Beijing girls’ school where she was deputy principal, a daughter of the Communist Party elite has offered public penance — of a kind that instantly brought controversy — for her part in one of the most notorious killings of the Cultural Revolution.

Growing numbers of aging Red Guards have declared their contrition for violence perpetrated from 1966, when Mao Zedong urged students to turn against the school and party authorities he accused of stymieing his vision of a revolutionary society cleansed of ideological laxity.

But the apology from Song Binbin, reported by The Beijing News on Monday, quickly drew attention and was featured on many Chinese news websites. Here was a daughter of a veteran revolutionary apologizing for what has been widely described as the first killing of a teacher in the decade-long Cultural Revolution....