And they allow the retarded brother to die, in order to bring back their friend from the dead.
Why does Data have to supplant B4? We're not talking about human beings here. Who says coexistence is out of the question? Or a blending of "souls?" Also, why assume B4 is retarded, or that there's even an android equivalent to retardation? Watching the movie, he seems child-like, undeveloped.