Beasts do not necessarily wish for humanity's destruction, they (usually) love humans (in a twisted way).
Kiara is more likely to destroy the whole world. In both MoonCancer and Alter Ego form, she's the closest to a pure Star Eater.
Space Ereshkigal describes Star Eaters as a type of "Bad End" rarely created by intelligent beings. Kukulkan describes Star Eaters as "shame" for their home planet that should be destroyed immediately.
Space Ereshkigal's description of Kiara Sessyoin is as a being who, using solely her own desires and lust, manipulates groups, poisons societies and rots civilisations, before eventually devouring the planet before emerging into space and consume the stars, leaving nothing but the sky. She might be considered the only person worthy of the class MoonCancer.
BB is THE MoonCancer by being an actual Cancer to the MoonCell
Arceuid is the princess of the moon (or something like that, I don't know Tsukihime well)
Jinako/Ganesha and Tenochtilan only got their classes through BB's meddling
Kiara thinks she got access to the class by "scarring the moon" during Fate Extra/CCC (and to spite BB), but Space Eresh thinks she embodies the principle of the class the most.
If Edison Alter is a playable Servant and keeps the MoonCancer class... I don't have enough info to speculate. IIRC, our Edison is only like that because he needed to be summoned to the 5th Singularity, and got his lion head from the concept of presidents (or some shit), so if this is an alternate reality were Chaldea never existed, then Goetia never tried to incinerate humanity, and this Edison wouldn't exist... So why would he still be a lion, and now an Alter, I have NO IDEA.
Ciel is also a mystery, I'm gonna wildly guess that she's MoonCancer because she got atomized by the being closest to the moon (Arcueid), and/or Servantverse shenanigans
Honestly I don't know.
