Mental ability

If people are exonerated for their crimes because they can prove permanent or temporary mental disorder, where can the line be drawn? People develop emotionally as they experience new things, meet new people, are exposed to incidence. Some advance further than others but when those who fail to develop, either through lack of opportunity or lack of will, can use the failure to their advantage after they have committed a crime and exempt themselves from punishment, who is responsible? They aren't responsible for their action if they are in the state where they don't know right from wrong. Should the rest of us be forced to accept this? Can the victims forgive them on the mental ability ground? Is there something wrong with that society? Should the society try to look after people's mental state? Should we renounce our personal responsibility to act responsibly? What happens when the society fails? How can we help children develop emotional competencies at home and school? Should parents be the main source of children's mental development rather than institution?