Just so we are clear, Diablo is the ENEMY and you don't play anything but a good guy, I have found that while rife with blood and gore, this game has a clearly defined sense of good vs evil. I think the OP may have been just hoping for an interesting POV
from players. This is a most diverse group we have here so I wouldn't go asking the OP to question his/her own beliefs just for asking.
Doesn't it all come down to this:
Killing demons = not getting in Satan's good graces
if you have to ask this then your beliefs got some serious issues.
there are more pressing matters in the world to worry about then a video game.
maybe ask yourself if you should be using a computer or a smarty phone... you know that smartphones are the mark of the beast... you hold them in your hand to your head they have your number and you will need them to buy and sell once we become cashless.
I read your headline wrong, I thought it read "should a christian be worried about playing this game?" To clarify you should play the game to your heart's content without feeling guilty. You aren't "sinning" by playing Diablo :P
Shouldn't your lifestyle be about the decisions you make in relation to your beliefs as opposed to what media you use as entertainment? Too often people are judgemental in regards to videogames, movies, music, ect. Your playing of this game isn't harming
anyone or affecting anyone (yourself included) negatively so I don't see why you should feel guilty. Do what you like and don't worry about the judgement or standards of others.