Well, considering that when you commit an action in a video game you’re not actually committing the action, but simulating it, you’re not actually sinning. Arguments could be made that it’s imprudent or has deleterious effects on the minds of some people. But you can’t sin by doing something in a video game, or at least you can’t incur the guilt you would by having committed the sin in actuality.
There are some sins which are simulatable, such as murder, which are permitted in plays, movies etc. to advance the plot. But, there are unsimulatable sins, such as blasphemy, which would not be permitted.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.