Well, if they get caught, then they get punished. If they don't get caught, but they regret what they have done, then they will be punishing themselves until they die, for the rest of their lives. As to where they go after this life, anyone who murders someone else generally, according to the scriptures, reappears in a "state of deprivation, in an unhappy destination, in ruin, in hell, or if they come to the human state are short-lived wherever they are reborn."
Now, in this question the person wrote, "ignorant criminal". In the court systems, they often try to decide on how sane a criminal is. The more sane they feel the criminal is, the more they feel the criminal planned the murder, that they actually schemed, that they actually figured out how to get done this and that. The more they considered that the criminal was sane, then usually the harder the penalty they give. If they actually feel the criminal was insane, the ignorance was so much that it's insanity, then they normally put this person in a sort of mental hospital instead of a jail.
So even a court system will judge how much punishment is given by considering how ignorant the criminal is. That is to a certain extent similar to the Buddhist understanding of the Law of Kamma. According to Buddhism anyone who kills is ignorant, but the Kammic result of that action will depend on many different aspects such as: how much they actually understood their ignorance; what their intention was; how much they planned to kill; how much harm they wanted to cause and so on. All those different aspects will determine just what the basic Kammic punishment will be, whether it's in this lifetime or in a future life.