Lol. well. Try to think of the lyrics you are about to create are from like... an old horror film or something

Death (obviously) Write a song about zombies
Write about killing someone
write about necrofilia :P
Anything gruesome is fair game..
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I'm sorry, but he said deathcore/metalcore.

Write about your feelings.
Seriously though, BMTHTaylor got it right. Horror, death and zombies are all fair game.
Picture every time you were ever hurt in any way and think of the deepest, darkest things you would do to the person who hurt you if you could get away with it. Get away with it through your lyrics.
Write about killing emo kids. Cuz its the scene thing to do.
Peace be thy journey
think of all the people that piss you off or some arss hole that was a prick. remember, don't harm people in real life duh, but, you can do anyting in a song! i kill people all the time in song. or you can go a different route of course and sing about god, he is pretty brutal if you think about it.
Just go "breebreeebreebree".

Seriously though, use lots of violent imagery.