I would suggest that a game crosses the line when you begin to do things to other people that you wouldn't do in real life and you wouldn't won't done to you.

I'm not much of a gamer, the only games I have are Chromium and SuperTux (free). For me, it's one thing to jump on a snowball character and squash it, it would be another to do the same thing to a character who was a simulated human being.

I may have more thoughts, additions, corrections later