Well, and what then?
Is your question if this pondering is sinful? I do not think so. It is a thought experiment that involves no actual intention to act out the thoughts, and it can never hurt to ponder your own reaction in extreme situations. After all, we might all be faced with a dilemma that involves killing or allowing yourself or others to be killed, choosing the lesser evil.
Another danger, which hasn’t been mentioned so far and that I would, personally, in some instances consider sinful is the possibility to invest too much time and energy in games and get lost in a fanstasy world, lost for real life. For example, playing WoW for as much time as you can spare and write fanfiction about WoW at work and constantly longing to be a character out of that world, instead of living the own life. But this is not limited to games, books and films have the same effects, I guess there are really women who get “lost” in this way through Twilight or similar books. Or people who are the same with Star Wars/Star Trek/whatever without ever playing a computer game with this theme.