Rubilax is very principled. He won’t kill Nava just because she wants it that day. His sense of self-importance is above that. Killing someone because he himself wants to? Yes. Killing someone because they ask him to — without even begging? No. Absolutely not. He is the authority here, and he decides whether the being in front of him lives or dies.
In Nava’s case, she has become useful to him. He won’t kill her. If she keeps annoying him with requests to kill her, Rubilax will beat the mare half to death — but in a way that ensures she survives.
Anyone else who asked Rubilax “kill me” would be dead on the spot that very second, so don’t think that would save you. Only Nava has plot armor against this monster, haha :З