I killed huge sums people for no more justifiable reason than I was bored and could. I did this over and over again, and in the process of each murderous body-strewn rampage I stole cars, caused billions of moneys worth of collateral damage, kidnapped people, aided in the drugs trade, exposed large numbers of innocent people to extremely dangerous conditions, and then some.
And that's not even the worst of it, I once killed the entire population of humanity by having them poo uncontrollably to death, while blind and insane, covered in sores. Technically I didn't do it, a designer strain of bacteria did, which I created.
Then when I'd finished doing all that, I took out some time for myself, sat back in my desk chair, and seized domination of an entire galaxy, committing act after act of genocide against peaceful species, wiping out ecosystem after ecosystem, spearheading a galaxy-wide slavery, drugs, and violence fueled galactic empire, void of freedom, rights, or even death.
Games I referenced:
GTA:SA, Saints Row 2, Pandemic 2, Galactic Civilisations.
Saying that, if there is a properly viable option to play using altruism, kindness, benevolence and justice, I tend to be heavily swayed that way. The first two aren't fun if you don't play a complete dick, and the aim of Pandemic 2 is to exterminate humanity... I've only ever played a really evil civ on GC once.