Riddle me this, readers

Given that Commander Blake was perfectly willing to nuke a spaceport and its surrounding city just to kill one kid, can anyone come up with a plausable reason why she wouldn't/couldn't kill Mel and the rest of her crew after she hands over Rufus and Hazel?

Cuz I can't figure it out myself, and I'd rather not kill Mel and Rollie, given I want the climax to actually be upbeat.