When in need of an evil Egyptian god for fiction, while Set certainly has a worse reputation, most authors go for Anubis. It makes sense: the Jackal God is popular, got a super-cool look, and I can find tons of visual references.
And then, hey, he’s the God of Death, right?
I can quote a lot of resources in that sense, from Roger Zelazny to Johnny Quest by way of Young Sherlock Holmes, Bram Stoker and Valerie Leon.
Fact is, it doesn’t work that way. Anubis, aka Anpu, aka Inpu, sometimes also known as Hermanubis, is the protector of the souls of the dead. He’s not the bad guy, he’s with the good guys! Let that sink in, and then tell me again why fanatics with daggers should serve him.
And really, apart from the philological elements, Anubis as the dark god of Egypt’s been done to death. Which is, I realize, somewhat ironic.

So, when outlining AMARNA, I looked up a few other Usual (Egyptian) Suspects.
And for my money, you want a bad guy in Egyptian myth? Go for Apophis. The Stargate SG1 guys got the snake right.

 

The good thing is that Apophis also goes by the name of Apep, so that I’ll distance myself from SG1 (of which I’m a fan, but that’s another story).
Quoth Wikipedia…

Ra was the solar deity, bringer of light, and thus the upholder of Ma’at. Apep was viewed as the greatest enemy of Ra, and thus was given the title Enemy of Ra, and also “the Lord of Chaos”.

Bingo.
The guy was obviously the soul of the party…

He was depicted as a giant water snake, almost always shown being pierced by knives or other weapons or under the control of a deity, so his image would not give the demon power. He was thought by the ancient Egyptians to be over 16m long, with skin as hard as flint. His roar was so loud that it shook the underworld. He was called ‘Evil Lizard’, ‘Opponent of Ra’, ‘Enemy of Ra’, ‘World Encircler’ and ‘Serpent of Rebirth’.

And during Roman times, Apep’s name was thought to mean “He who was Spat Out”.
It’s good to be popular, what?
Oh, and talking about popularity, consider that Apep was never depicted if not being smitten or cut or in any other way restrained – just to be on the safe side.
He was often killed by a cat, in the paintings – the cat representing Ra.

Which leads to the thing that really sold me on Apep as the right god for a fictional death cult: the fact that Apep was worshiped against. Meaning he was not historically object of a cult, but there were specific rituals performed to keep him out of the way, including “the spell of spitting on Apep”, “the spell of trampling on Apep with the left foot”, “the spell of taking the spear to smite Apep”, “the spell of binding Apep”, “the spell of taking the knife to smite Apep”, and “the spell of setting fire to Apep”. Red-hot knives were said to work better when cutting Apep in many pieces.

So, coming to the world of fiction, just imagine the sort of people that could be attracted by a Cult of Apep.
And consider what charming guys probably where the two pharaohs that during the 15th dynasty took their name from this charming guy.

Food for great stories.