No, I don't think it makes you a bad person. If it's a one-time thing, I doubt half a beer will make any difference.
It's a personal choice, one that some people feel very strongly about. I know some women who have an occasional glass of wine while pregnant and their kids turn out fine. Plus, a lot of women don't even know they're pregnant until they're 5-6 weeks, and who knows how much they were drinking before they found out.
If you are thinking about drinking again in the future, do some research about the risks so you can make an informed decision about what's best for you and your baby. It's a very personal decision, and whatever you decide, I don't think it makes you a bad person.