I definitely don't have any beliefs in satan or god, that's just irrational. The only true "evil" as some might call it is within all people, and metal is a genre of music that glorifies and embraces all parts of the human psyche. I wouldn't say I randomly have the urge to go set people on fire, but we all have violent tendencies and I prefer the honesty of metal and enjoy that aspect of the music. It's not intended for people that are looking for more ways to deny their humanity and fit into what has become acceptable etiquette. Which in most of America at least, has become christian standards for control combined with expectations and opinions enforced by general media.
Besides the more philosophical meaning though, I listen because it sounds fucking awesome. To me an awesome growler or a great guitarist is a more preferable sound to hearing someone being a douche on American Idol or a pop CD.
Which really brings up image, because most people in pop music are at least partly judged on their looks. I saw a video review for some music magazine on youtube where a britney spears album was knocked a few points down because of her "image" and behaviour. Not 100% why I saw this video honestly so don't ask. Seriously, you don't see bullshit like that in metal. You can be one ugly motherfucker and fit right in, and in fact stupid behaviour is expected. They day someone pulls 10% from a hypocrisy album because Peter looks like several aliens combined with Johnny Depp, or Amon Amarth gets docked points because Johan Hegg is several centuries behind in fashion is a sad day indeed.
As for devil horns... Well that just the symbol of metal really. At a concert I do it to show I'm loving the show sometimes. There is no relation to satan within the symbol anyways, that's just its adopted meaning. Metal's image in more rational people (not grim and trvv people) I would say is even anti-image. It's just showing up for a concert and acting as retarded, brutal and crazy as you fuckin' want and there is nothing wrong with it.