... for fun I guess? Personally if playing evil it's Lawful or Neutral, using society's systems to my advantage or just being selfish respectively. Or because a cool class is locked behind morality i.e. Assassin, Blackguard or Necromancer.
Though if thinking outside RPGs and morality as a physically manifesting concept, many games lets us do things we'd never want to or be able to in real life. Saints Row/Grand Theft Auto is extremely fun; being able to hop into any vehicle and shoot anyone who is an asshole in the face is extremely satisfying. But I'd never wish to steal someone's car RL and there's not many people I'd risk jailtime for for removing from the populace.
