The thing is, if you have two rival religious traditions in juxtaposition with each other, they are fairly likely to accuse the other of being devil worshipers. We don't have to look very far--many modern Christian fundamentalists aren't reluctant to point fingers, at Hinduism for instance. Or of course the Protestants of the Reformation age charged Catholics with following the Whore of Babylon...
Rivals don't always do that, but it isn't uncommon. Imagine for instance if the Nordic pantheon, or the Slavic one, had somehow survived and prospered in the face of Christendom, surely the Catholics would say (as they did before and after their OTL triumph) that the Aesir or the Slavic gods were in fact demons; the alternative being to deny they existed at all of course. Perhaps the pagans would reciprocate and find "evidence" that the Christian God was really some demon figure from their own tradition, perhaps modifying Loki to fit, or manufacturing a new fallen god to match.
One can imagine going beyond mere name-calling and that, in the face of a dominant religious tradition, a cult defiantly adopts the label of "the Evil One." Many traditions include the idea of "the Trickster," such as southwestern Native American "Coyote;" perhaps Loki more authentically played a similar role in Aesiric tradition and was reconfigured into a more clearly demonic role by the Christians who recorded the Eddas. Indeed Satan himself, who first appears in Job, is there represented not as the demonic enemy of God, but as a servant of God's who was created to test and challenge God's creations, to test to destruction if so authorized in fact. Hence the title "the Adversary;" he's more like a very zealous prosecutor than The Enemy. A pantheon with a Trickster God has somewhat different values than the "No Darkness in the Divine" world view of orthodox Christianity; when such black-and-white religions look at the deeds and implicit values of such religions they don't have to look far for evidence that the gods and values have "evil" in them and thus must be evil through and through.
But even defiant rebels who adopt the banner of the allegedly evil Trickster tend to believe themselves to be the real good people, embracing chaos and controversy to overthrow stagnant oppression. Can we go farther and imagine a serious cult of evil itself?
If some, for whatever reason, began to believe that their religion's forces of evil were more powerful or more active that the forces of good--or just that the good diety wasn't always watching, they might honor the evil one so he turns his wrath or mischief elsewhere. Build the devil a house/temple, and he won't move into YOURS...or help your enemies.
This seems entirely apropos!
But to get "real Devil worship" I think we have to go farther still, to imagine people who don't just cower before unstoppable power and hope to deflect its ravages somehow, if only by self-destruction, but who embrace values that are clear and consistent inversions of normal human ones. Who uphold cruelty and lying and destruction for its own sake as paramount values.
And there too I fear we won't search in vain. Some of the kookier neo-Nordic cults of the Nazis, with an eye toward Nietzsche's scorn of traditional religion as a mere "slave morality," seem to fit the bill. Modern Neo-Nazis sometimes take the idea even farther.