1) Objectively, in the global scheme of things Red Court is far more of a danger, and Arianna Ortega is more important politically than a single naagloshii
Arianna Ortega is just one vamp.
She'll cause damage, but sooner or later she'll get killed either by her enemies, or her rivals, or her subordinates.
Or just in the fall of the Red Court in general.
There's a limited lifespan to her evil, despite her theoretical immortality.
Her kind of person makes her death a good thing for too many people to really enjoy the longest run.
Meanwhile the Naagloshii will hunt, torture and kill people for a very long time to come, and if he doesn't anger an Exalt, Mab or someone capable of commanding Demonreach while doing so, he will continue indefinitly.
So I think all in all Seeker will cause more harm than Arianna will ever have a chance to cause.
2) Eliminating Arianna benefits us more than eliminating Broken Seeker
Propably true.
All Broken Seeker has going for him is that an old demigod is good crafting-mats.
3) Chance for redemption. Broken Seeker sucks in social combat, as has been just demonstrated. Reforming it, or at least setting it on the path of reformation, might work
Not worth it.
Ancient monster, had his chance.
Only reason he's less bad than Nicodemus is because Seeker never had large ambitions to mass-murder for, so he staid a regular serial-murderer.
4) Opportunity. Nothing says we can't eliminate both, with Arianna first and Broken Seeker second.
Maybe, but that's a risky game.
Neither of those is stupid, even if they can't contest us in direct socials.
Ultimatly Arianna is someone we can take even without a Naagloshii as help, we don't need this deal.