If you consume negative media eventually your mind will conform to the negativity.
If you consume positive media your mind will conform to the positivity.
If you feel like said media is bad for your mental and spiritual health, it probably is.
Bear in mind that there’s a pretty long history in Catholic art of using morbid imagery and telling haunting tales, and this goes all the way back to Scripture. There’s clearly nothing inherently wrong with such content, and most people who listen to metal are more than capable of processing it healthily.
The problem is that at least since the “Satanic panic”, there’s been so much written about how “Satanic” metal is that Christians who may have never had a problem and are more than capable of processing it may have doubts. To quote Joseph Goebbels: “If you tell a lie big enough and keep repeating it, people will eventually come to believe it.”
Personally, I’ve found a much better test is that if the song starts clearly leading you astray, generating ill-will towards the sacred, distracting from times of prayer, etc., then it might be best to cut it off. Most cases, though, I’d say recognize that you might just be internalizing the lies that are often told of the genre and many bands in it.
But at the end of the day, plenty of Christians have been able to listen to Slayer and similar bands for years without it leading them astray. Heck, Tom Araya has had to sing that stuff for years, and he’s remained a practicing Catholic last I heard.
Slayer and Black Sabbath are Blatant about what they believe
Tom Araya (Slayer) is Catholic, has stated multiple times that Slayer isn’t Satanic, and said that the imagery was always meant to scare people and separate themselves from other metal subcultures of the time. It’s never been a matter of Satanic belief. The same would go for other “Satanic” bands like Judas Priest, Black Sabbath, and (most laughably) Iron Maiden.
In fact, I’d wager that most metal bands that have such an image don’t actually believe in any of it. There are some exceptions, but unless the band is open about trying to be blasphemous (e.g. Ghost) or Satanic (e.g. Watain), it’s always best to default to it being at most an act. When you consider that a lot of metal lyrics are based on fantasy (including dark fantasy and horror) or shining a light on darker aspects of humanity, it recontextualizes a lot.