If anything is traditionally ubiquitous in heavy metal, and not just extreme forms of metal, it’s any kind of association with the devil. Since a lot of heavy metal bands mention Satan in some way, it’s been assumed they are of the devil’s camp, even though it’s often done to be cool. Because of this, Satan has become a kind of cultural symbol for the world of heavy metal. A lot of people don’t seem to realize the diversity regarding how Satan is dealt with in heavy metal music.
There’s bands that sometimes sing about Satan but they’re just rocking out and trying to write cool songs. Examples I can think of include Exodus, Demon, Nuclear Assault (on their first album at least), Witchfinder General, and, in recent times, Enforcer.
There’s bands that do not frequently sing about Satan at all, but are still associated with Satan because of either the odd use of the Satanic pentagram or because conservative types don’t know any better. Examples I can think of include Motley Crue, Iron Maiden, Megadeth, and WASP. This also happens to hard rock acts outside of metal, such as KISS, Alice Cooper, and Ugly Kid Joe. There’s also plenty of metal bands that use devil imagery but are not really attached to Satan at all, and there’s too many of them to count. There’s even a band called Satan that doesn’t have a whole lot to do with Satan.
Then there’s bands where Satan is the whole point of their music, but not necessarily Satanism. Here Satan in the traditional or just rock and roll context is celebrated and central subject matter, often alongside other traditionally dark or evil forces. Examples I can think of include Venom, Mercyful Fate, Destruction (early on), Sodom (also early on), Bulldozer, Possessed, Onslaught, Infernal Majesty, Midnight, Speedwolf, and Ghost (aka Ghost BC), along with many black metal bands both old and new. I’d also like to mention that Running Wild used to sing a lot about Satan and demons until 1987, when they released Under Jolly Roger, when they started concentrating on writing songs about pirates and history, and in those days Satan was often written about as a heroic figure who brings freedom and peace to the good and destroys the worst of people. Also, with full-on black metal bands, not all of them are all about Satanism, and not all black metal musicians are actual Satanists or promote real Satanism. There are black metal musicians who are actually Satanists, and even then not all of them promote philosophies that can actually be called Satanism.
There’s also bands that, when they do sing about Satan, they sing about Satan from a negative point of view, as in they sing about how he’s supposed to be the bad guy from the Christian point of view or how he’s supposed to be defeated in those beliefs. Aside from so-called Christian metal bands (such as Stryper), there’s Megadeth, Hirax, Cage, Trouble, Pentagram (ironically enough), and Black Sabbath. Black Sabbath, the band that would come to be known for laying the foundations for heavy metal, never intended to use pentagrams or upside down crosses, nor did they intend to glorify Satan. They had an interest in the dark side, and an interest in creating dark and heavy music, but it was also a kind of fear of the dark side, and their style of music was intended to reflect the reality of the environment in which they lived in during their youth (at least until Ozzy Osbourne left the band), in contrast to the flower power being expressed in America.
Finally there’s the that come close to actual Satanism in terms of lyrics, in terms of actual veneration of Satan and sometimes actual philosophical Satanism, often mixed in with defiant anti-Christian sentiment. Examples include Behemoth, Morbid Angel, The Electric Hellfire Club (although they used to be just an industrial band), Root, Deicide, and Acheron. King Diamond of Mercyful Fate is also a genuine Satanist, but the songs he writes for both his solo band and Mercyful Fate are more like stories of devil worship and horror, though I suspect some proper occult context is vaguely applied in some songs. Also worth mentioning outside of heavy metal are bands called Coven and Black Widow. They are not heavy metal (Coven was psychedelic rock band, and Black Widow as a folk/progressive rock band), but they gained fame before Black Sabbath and before any heavy metal band for associating with overtly Satanic imagery or writing songs with a Satanic or occult themes (though Black Widow soon abandoned that direction), and their lyrics back then could make some later metal bands lyrics look pale or insincere by comparison.
And that’s basically it. All in all I really think the whole idea with Satan and rock and roll of any kind is to just have fun with it. A lot of times it’s ultimately shallow and you can only really take it at face-value, and some times you find genuine Satanism, but I think all you can do in the case of music is just have fun. If lyrics about Satan are fun for you, great. If not, you can always look at other songs. But if you care so much about real Satanism being proliferated in rock and metal music, then I’ll say what Alice Cooper once said: if you want real Satanism, don’t look in rock and roll.