Since this is a bass forum after all:
And (most) metal does not condone violence, in the same way that Stephen King doesn't. They just thematise the darker emotions like fear, anger and hate that every human carries inside them. What does a serial killer think or someone sitting on the electric chair ? Certainly more interesting and thoughtful than Miley Cyrus or whatever the shag bubblegum ******** is stuffed down your throat everyday.
Concerning vocals, I do listen to melodic death metal quite often, but I draw the line where I can't understand what the singer is doing. So Amon Amarth, Arch Enemy or Feared is totally fine for me, since they actually have great technique and diction. (Try it, its really, really hard to sing in that style and still form words in a way that is understandable.
Stuff like Cannibal Corpse or Entombed ain't really my thing, great music and very talented musicians, I just can't understand a word.