Most natives in Carcosa are illiterate, but sorcerers and other scholars have learned the writings of the Snake Men in order to study rituals and other forbidden lore. And while history and legends are generally passed down orally, some vainglorious chiefs and heroes may wish to have a scribe record their deeds for posterity (for anyone with the skill and inclination to read about their exploits).
Human scripts would be derived from Snake Man alphabet, modified according to their needs.
Some sorcerers have been able to decode the alphabet of the Old Ones in order to help them learn more rituals. B'yakhee, Deep Ones, and Primordial Ones understand and use this script.
Space Aliens are civilized and have their own alphabet. They doubtless keep detailed records of their civilization and their experiences on Carcosa.
The way I see it the Space Aliens are post literate, communicating mostly by telepathy (both natural and electronic). If information must be presented in graphic form, it's done by illustrations and autoplaying holograms; this is the reason most Space Alien technology is so easy for the Men of Carcosa to use.
Any fool of a DM can kill his players, it's as easy as "Rocks fall, everybody dies". The Art lies in giving them the opportunity to kill themselves, in as entertaining a manner as possible.