Your GM doesn't seem to want to kill off the NPC, so reasons why your character wouldn't want to:

-Is Catherine a magic user? Perhaps the werewolf girl lets it slip that she's got some modest magical power, and You've Got a Bad Feeling About This... something that old and that strong is going to have one HELL of a death curse. Perhaps another faction purposely set up the Rukh as a booby trap, and is counting on her death to cripple the Venatori. Maybe not, but your sense is telling you something. I like this solution, because if there's a threat to the Venatori from killing the vampire, saving her makes sense for all 3 of your characters.

-How much does your character know about the frame job? Perhaps Catherine smirks and makes an offhand comment, which makes you realize she may know about who was actually behind it. If that's a strong enough motivator, maybe another option. Having her dangle any tantalizing information that's important enough for your character to take pause at hearing may be a good enough motivation to axe the execution, or at least delay it.

-Your queen compels you to save the Rukh; unbeknownst to you, there's a great deal of political intrigue going on, and while Summer isn't usually on great terms with the BCV, the Rukh dying would really screw with Summer's plans.
Cold Days spoiler:
-Hell breaks loose, the site is under attack, and a traitor in the Venatori (maybe even the one the Werewolf girl hates) has sold them out to an outside faction; in the ensuing struggle to survive, the Rukh escapes. This will only work if your GM is fluid enough to rethink his story a bit.


The other consideration is asking your GM what major plans he has involving the NPC. If you're going to be railroaded and you want to work with him, let him at least tell you what bounds he needs you to fall under.