I generally set out from the beginning with a certain role. Most often, I am the good guy, trying to do the right thing. But, if I've played the game a lot, I'll give chaotic evil a spin.
Reminds me of the game Fable (I haven't played 2 or 3, just the first one). My first time through the game, I played the hero at every choice. Near the end, my shining-white angelic paladin character moves, leaving butterflies in his wake, literally. Second time through, played the villain - and the character turned as ugly as his actions, horned, dark and malevolent - leaving bloody footprints. Nicely done.
I loved Fable, despite Peter Molyneux's over promising, but I always find games with alignments to be too simplistic. For instance in Fable 3 (non-plot realted spoilers ahead) at one point you can choose to instigate a sort of one-child policy (evil) or encourage people to have children with government support (good). Later you get can legalize child labour (evil) or fund schools (good). I of course chose to encourage child-birth, while legalizing child labour and having taxes and rent across the kingdom so high that I would have a massive, expendable, cheap labour force. My now-wife was a bit shocked at my penchant for evil in games, especially since it so starkly conflicted with how I acted in real life. IMO, games where there are no real-world ties (i.e. non-multiplayer games) would be a bad source for data on morality.