Okay, having gone back and reread the relevant comics for this story arc, this is all pretty misleading? The way it actually goes down is that the main OT cast makes an alliance with the Partisans in order to sabotage the Spike, a key piece of infrastructure on Shu-Torun whose loss would cripple the industrial facilities that the planet has provided to the Empire. At no point does anyone ever say that the planet would become uninhabitable if it were destroyed. The Partisans try to change the plan and sabotage the Spike in a way that would actually devastate Shu-Torun in revenge for the destruction of Jedha and Alderaan, but Luke and Leia manage to talk them out of it. Also, it's not like Leia just puts a blaster bolt in the back of the Queen's head execution-style—it's a fight, and she happens to deal her a mortal wound in self-defense.