Yes, the evil soldiers are possessed humans.
I don’t remember exactly where it was explained; possibly in the manual, possibly in some FAQ. But even the sprite names used for them in DOOM.WAD make it clear:
Pistol Soldier – POSS (Possessed)
Shotgun Soldier – SPOS (Shotgun Possessed)
Chaingunner – CPOS (Chaingun Possessed, DOOM2 only)