I kinda feel like we might be overreacting to the black text a little bit. Black text doesn't mean black light any more than red text means red light. The poor man studied to be able to feel the souls in the world around him, and he has now had a brief moment of existential crisis from witnessing a form of death so complete that it leaves no sign that the victim was ever alive.
Especially since neither really fits the description.
The Anti-Life Equation has never been linked to a religion on Earth that I can recall. It was implied that some demon worshipping Gothamite witches were actually worshipping Darkseid by mistake, but that's the closest I can think of.
Nor can I recall Nekron ever being worshipped, other than Black Hand his interest in people is killing them.
Yes, a druid learned how to manipulate the black light, but I'm pretty sure if the characters meant druidism they'd say druidism and not "dead religion."
"Dead religion" is code for a hipster religion no one has ever heard of, not something pretty much everyone in the western world has heard of.
Honestly, the "no echoes of their life which should be there even in death" sounds like soul eating to me. The black light doesn't make people deader than dead, or else that would happen to everyone, since Nekron runs DC's waiting room for souls, an aspect that Zoat has said he's keeping in this story just recently.
And anti-life is all about emotional manipulation, in that case I'd expect Question to pick up loneliness, alienation, fear, despair, mockery, condemnation, misunderstanding, guilt, shame, failure, and judgement, not nothing.
Combining soul eating with religion, in the DC franchise, the first person that pops into my mind is Trigon. Who has been worshipped as a god, and has been known to eat people's souls as a power up. When trapped magically, he freed himself by eating every single soul in his native universe.
DC being DC, there are probably other beings that have been shown to consume souls and be linked to a religion.