Hmm, I've never really thought to articulate it, so you've kind of got me at a loss. I'd say when I was first getting into metal and I still felt some attachment to the "spiritual world" or whatever, it felt dangerous and rebellious, and maybe my lingering attraction to it is somewhat of a vestige of that time in my life. Now though? I think I just like dark, evil stuff for the same reasons I like, say, superhero comics and Western movies. It's an escape from the familiar. It has nothing to do with my life, but what it presents is "cool" to me.
This is ridiculously hard to put into words, sorry for rambling.