On the matter of the band Slayer - if you take a look at some of their songs and accompanying lyrics, they're more anti-religion than pro-Satan.
Here is a quote from their guitarist Kerry King, taken from the documentary Metal: A Headbanger's Journey:
"I like to take potshots at religion because I think it’s the ... biggest brainwashing thing that is totally acceptable in America and probably most every place else in the world. Um, I just … I think it’s a load of [expletive]."
Whether or not anyone else here agrees, this is what Kerry had to say, so Slayer is in fact against organized religion instead of promoting Satanic imagery. They had songs criticizing not only Christianity ("Disciple" from the album "God Hates Us All" and "Cult" from "Christ Illusion") but also other religions (the song "Jihad" from "Christ Illusion", which is a dig at Islam, though I must admit those lyrics were very misinformed and badly researched from an objective point of view). Their songs also portray a lot of grim stories about matters somewhat, though loosely related to religion, such as "Angel Of Death" from "Reign In Blood" that was about Josef Mengele, the Nazi doctor that performed brutal and inhumane pseudo-scientific experiments on Jews.
About Metallica, one of their album covers portraying what we commonly envision to be Satan is news to me, but not surprising. Did you know that the album covers for their consecutive studio albums Load and ReLoad were made by setting sperm on fire? They consider themselves artists, and as musicians that play in probably the darkest niche of music since the likes of Wagner and Stravinsky, they like to do some dark stuff. My guess is that the Devil would definitely qualify as "dark".
The general rule of thumb when it comes to analyzing metal or rock that I like to stick by is that if you see Satanic imagery anywhere except the subgenres of black metal or death metal, then it's intended for shock value or part of a one-off statement about Satan they just happen to want to express. There's a pervasive competition in the metal culture between bands of any subgenre, the goal being to sound more evil and dark than any other band. Rarely do these bands get to endorse Satan even if they wanted to, because the overwhelming majority of record labels know that endorsing Satan doesn't sell well.
But in death metal, there is an unhealthy obsession with everything dark and morbid about life, and that means death, the Apocalypse, the Devil, other versions of the end of the world, crucifixions, torture, obscene mutilation of body parts, perdition, and various other atrocities I won't get into. When these bands reference Satan, it's usually to glorify him, and I must admit it is difficult to distinguish which ones are doing it as a publicity stunt or which ones are genuine.
In black metal, there is the most Satanic imagery. But that's the result of a trend that was started in Norway, not necessarily on purpose. As previously mentioned, there were the church burnings in Norway, and it just took off from there. It's a curious phenomenon - the original black metal bands were simply against anything that had Semitic roots - this included all Abrahamic religions. For some reason, later bands began to adopt Satan as master, lyrical content, and his imagery. This is ironic considering Satan came from the Abrahamic religions. And an interesting tidbit about Norwegian black metal bands - some of them are particularly against Christianity because of the perceived unwelcome Christianization of Norway centuries ago. They prefer their pagan ancestry - Asatru, Odin/Wotan, Yggdrasil, Valhalla, Asgard, Niflheim, all that stuff, and they felt that their ancestors did not have a choice in the matter of religion, that Christianity was forced upon them, so in their eyes the church burnings and the anti-Christian sentiment is only natural.
