The more intellect a person has the more he/she is responsible. When the intellect is missing, the responsibility is also not there. Little children are not held responsible, because their intellect has not yet developed. The insane are not responsible, because they have lost the intellectual capacity. However, part of our being human is also that we make mistakes. Sometime we make mistakes without deliberation and intention. But sometime we knowingly and deliberately sin and do wrong to others. It is said, “to err is human and to forgive is divine.” Both parts of this statement are very true. As human beings we are responsible, but we do also make mistakes and we are constantly in need of forgiveness.
source - http://www.islamawareness.net/Salvation/forgiveness.html