In some context Set is also represented as helping Re cross the barqe of eternity to journey to the underworld. He becomes a savior slaying the serpent of Apep[ignorance],so in all accunts Set is not necessairly evil. But in most instances Set is ineed a representation of chasos that came to represent the Western Desert.

But I too wonder if He is a god, or if he is a devil or a demon.

My understanding is that originally he was a god and a celestial deity.
Some points are;
1) He was given the title "Majesty" like Re.
2)Asses were sacrificed to him, like horses sacrificed to Helios.
3)Rameses II made sacrifices to Set in order to make fine weather.

Devil;
1) Described as "great serpent of the Netherworld" and so is the World Serpent of Nordic Mythos- an evil devil.
2)Set kills Osiris.

Demon;
1)Se-Tekh can translate as "One who causes drunkeness". So is like Bacchus a daimon in Greek Myth.
2)Primitive cult to Set; may have even contained human sacrifice?

I also wonder if he is like Lucifer; a fallen Celestial. Once a Sky-god but later a chthonic demon. Seems that their was a change in the attitude to this god. First worshipped but later discredited and seen as evil.
The Hyksos worshipped him as Pep- a serpent and a dusty whirlwind. Hyksos were a lower race than the cultured aristocratic Egyptians.

As far as Set accompanying Re in his barque through the duat each night, since Set was the personification of evil, the idea was to use evil to overcome evil(demon, fiend).