Excuse me while wax philosophical here….
If there were truly no consequences for our actions, then that would necessarily mean that there is no sense of morality or immorality, thus it would be impossible for anyone to do evil… or good.
I think, however, what this question really wants to know is what immoral thing you would do if you knew you could get away with it. And for me, it would be nothing; I would do the same thing I’m doing right now, even if I had some sort of legal and spiritual get-out-of-jail-free card.
Jewish philosophy has a very intriguing idea. It is said (metaphorically, of course) that when you are in the womb, an angel teaches you Torah. Right before you are born, he smacks you on the mouth and you forget all of it. But throughout your life (provided you’re a normal human being), you have an innate sense of right and wrong, morality and immorality, justice and injustice, truth and falsehood, precisely because your subconscious mind has some memory of God’s law that you once learned.
For example, normal human beings are inherently opposed to incest and cannibalism. Why? Yes, in-breeding is genetically disadvantageous, but it’s perfectly possible in this day and age for a man to sleep with his mother or sister and never produce offspring by either. And yet the vast majority of family members do not have sex with one another, and when they do, we label them abnormal and put them in jail (why is incest even against the law if both people are old enough to consent?).
I was just reading on Slate.com that the average human being can produce 66 pounds of edible meat, plus protein-rich bone marrow. There are some health risks associated with cannibalism (kuru or prion disease, which is the same thing as Mad Cow), but these can generally be avoided if you do not eat the brain tissue or nerves. And yet most people have an in-born distaste for the idea of cannibalism. Even in cultures where it has been practised, it was almost exclusively done for ritual or religious purposes. No mentally-normal person has ever raised or kept humans solely for the purpose of eating them.
Even people who identify as atheists–who truly believe there is no cosmic or spiritual reward or punishment for their actions–do not practice incest or cannibalism, and would never do either, even if it was legal to do so. Why not?
If you believe that every mentally-normal person is born with some sort of innate sense of right and wrong–whether God-given or genetically evolved–then no normal person would be truly evil, even if there were no consequences to their actions–if only because you can’t override your conscience that much.