Larxene was fire. She was anger (Xigbar will vouch for that) and passion (that would explain the satisfied smirks that Roxas and Demyx shared) and hatred (Xemnas had many reasons to give Larxene missions, among them her loyalty to her own dark-flamed not-heart).
Larxene cackled and crackled and threw up sparks when she was excited. She burned higher with more fuel (blood, screams, pain, death) and refused to be soothed, to calm down, to dwindle and fade away, leaving behind nothing, not even ashes.
And then the day came when she couldn't refuse anymore. That day, her flame went out.