D&D 5E psychic scream: 0hp or killed

[seebs]

But... if the GM is playing, it, it is an NPC, by definition. In what scenario does the player not have this information?

Player knowledge and character knowledge are distinct. The special-casing of "dying" for "monsters" is one of the few cases where the abstraction leaks such that a sufficiently insane wizard can perform an actual test to determine whether something's a PC.

Wizards: Wall Of Steel, Fourth Wall Of Kleenex.