Y’Golonac is a brute, largely referring to physical appearance. If he manifests physically it is always as a large headless humanoid figure with mouths on his hands— interesting as he is the “a god of perversion and depravity”, not just of humans but all that can be conceived by any being.
The mouths may be a more symbolic interpretation of wanting to literally consume people, as well as knowledge of the morally reprehensible, as in this passage from The Revelations of Gla’aki (Chapter 12):
“… but when his name is spoken or read he comes forth to be worshipped or to feed and take on the shape and soul of those he feeds upon. For those who read of evil and search for its form within their minds call forth evil, and so may Y’golonac return to walk among men …”
Thus we may get a more literary interpretation of Y’Golonac as an unhesitatingly awful being who constantly pushes and breaks moral boundaries, who has absolutely no conception of limits, and who pushes others to do the same.
Something that becomes immediately apparent on reading an overview of Y’Golonac is that he is often compared to Nyarlathotep, such as in the below excerpt from Wikipedia:
“His demeanor is much like that of Nyarlathotep, but he is much more perverse and sadistic.”
This comparison may be hard to make sense of initially, but with some analyzation the reasons why may become more apparent.
The lack of information comparing them both was immediately frustrating, because several shared traits are obvious and could be easily explored. The Wikipedia article mentions their shared intellect in understanding humans and also refers to their demeanor, however, it does not go into detail on this.
Y’Golonac can be fairly easily summoned (just by speaking his name from the Revelations of Gla’aki) and this presents problems for those in lore who could easily accidentally summon the god. Upon summoning, he either offers the human the chance to become his priest, or promptly eats them.
Since Eihort and Gla’aki generally do not offer the chance of actual life upon meeting humans, Y’Golonac’s “sparing” of humans is interesting. While Nyarlathotep is not infamous for killing humans on sight, he also does not hesitate to use humans for an end goal at his discretion. Therefore we might reason that Y’Golonac has some goal in mind for his priests, or wishes to force a sense of attachment upon them.
He has an intense interest in those who seek awful, forbidden, evil material. This also shows some parallels with Nyarlathotep, though it could be argued that he more errs on having an interest who those who pursue universal knowledge (mostly as in cosmic knowledge), or those who seek the utmost knowledge of understanding of human psychology. Therefore their domains of interest and knowledge would differ.
Y’Golonac’s sadism is defined as being worse than Nyarlathotep’s, yet there is little point in going so far you kill your followers, especially since copies of the Revelations of Gla’aki are said to be so few in circulation.
So I perceive he may instead force his followers to pursue more forbidden material and doings, whether this be through forcing them to commit murders in exchange for knowledge (as in Nyarlathotep’s Church of Starry Wisdom), or of making them the witness of awful cosmic or earthly events (such as wars, torture, psychological breaks, etc.).
Another element of comparison that is interesting is their stature and personas. Y’Golonac only has his one headless form, while Nyarlathotep has several forms, some horrifying, others able to appear normal enough to not scare humans. The latter deity also has frequent references to a regal or semi-religious nature and symbolism attributed to him, such as in The Dream-Quest of Unknown Kadath:
“Then down the wide lane betwixt the two columns a lone figure strode; a tall, slim figure with the young face of an antique Pharaoh, gay with prismatic robes and crowned with a golden pshent that glowed with inherent light. Close up to Carter strode that regal figure…”
This characterization of Nyarlathotep indicates a juxtaposition between the frank expose and love of the macabre, morally horrific, and utterly sinful Y’Golonac displays, with the hiding of a dulled down form of this nature behind a royal or religious mask, as Nyarlathotep usually displays.
In other words, the contrast of outright accepting, embracing and comitting acts of sin constrasted to one who tricks his followers into thinking he is one of pure holiness through illusion. (Two sides of the same coin.)
Nyarlathotep makes frequent references to sin in his appearances in games like Persona 2 and Sundered. Considering Nyarlathotep is quite a manipulative being, it is not too much of a stretch to assume that he simply pretends have to morals and concepts of crossing the lines of sin, using religious motifs in order to manipulate humans.
Overall I believe Nyarlathotep’s followers would be more pious or loyal, simply because he is an excellent manipulator, and ultimately we lack some information on Y’Golonac’s manipulation skills. Despite this, followers of Y’Golonac would truly know they were spared from Britain’s other murderous Great Old Ones, and might feel some sense of allegiance toward him as a result.