Is ignorance a sin?

If I do not know the rules of a country, that does not count for that much in a court of law. I will still be punished for breaking the law. Likewise, if I do not know that bad speech results in bad karma will I be punished (in the sense of bad karma in the future )? Or maybe if I don't know what bad speech is will I be punished? To put the most extreme example, if I murder millions of people but I have a brain disorder and don't realize that is wrong, then will I be punished in the future by karma?