Carissa thinks that she just... rejects that frame, which Keltham likes to think in, for contemplating what he is doing.
Carissa does not actually feel that there is any unfairness in the fact that the entities who sealed Rovagug got more of what they wanted than entities which showed up much later and didn't seal Rovagug. That seems entirely fair to her. She doesn't think that "I'll help seal Rovagug, if you do X once Rovagug is sealed" is at all the same kind of conduct as "I'll destroy the world, if you don't do X which makes me not want to" -- a Carissa, obviously, would have helped seal Rovagug without conditions, but she's happy to work with entities of the first type and fundamentally thinks the second ought to be preemptively cryopreserved for everyone else's protection.
Carissa thinks that mortals have more of a seat at the table in Sarenrae's values, or Shelyn's, or Desna's, or Shizuru's, than in Keltham's; she thinks that those gods have something closer to the median values of a mortal in Golarion than Keltham does. She thinks that dath ilani morality is - askew in important respects from human values which make Keltham a particularly bad representative of humanity.
Dath ilani mostly are easy to get to no longer want to live. Golarionites aren't. Dath ilani mostly experience cognitive enhancement as personality death. Golarionites don't. Dath ilani mostly think about morality in precise theoretical terms borrowed from bargaining - fair shares, what mortals would have had if they'd helped seal Rovagug - and Golarionites don't. dath ilani don't seem to have the same care for tradition and history that Golarionites have. None of the things Keltham is saying are Golarionite things. He does not represent Golarionite values, and his having power would not make the world more of one in which Golarionite values, especially the extremely common Golarionite value of not being annihilated, would be represented.
Keltham is another alien entity who wants to destroy the world if it doesn't meet with his approval, and he is not an alien entity whose wants actually serve the people whose lives he is toying with. And while if someone ELSE went and released Rovagug this week she would be willing to make concessions to get Keltham to help her fight him, they'd be concessions to Keltham's wants, not to mortals' wants, and it's another thing entirely to say that he should himself get the rewards of having fought Rovagug if he didn't.