Storywise, I think that killing the netherbrain without subduing it first , will ceromorph all infected mortals and create a big army of mindflayers. When the netherbrain surrenders, you command it to destroy all the tadpoles before destroying itself. Without this, the effect would be like Gale blowing himself up in moonrise towers. The netherbrain is dead alright, but all the tadpoles turn to mindflayers. So, IMO, that should also be the logical outcome of instant-killing NB this way.
