Wouldn't any game or activity imitating breaking the law or doing things considered sin technically be sin?
If it is a sin for a man to look at a woman and lust after her, even if he doesn't do something, then I assume that
pretending to kill someone is sin too. Especially if it makes you feel agitated, excited, geared up ...
(side note, this is coming from a person that has played games like Diablo and The Suffering
)