(Warning: this post discusses a child killing case from the UK committed by a mentally ill teenager, and issues surrounding this such as stigma and failings in mental health care.)
Diminished Responsibility is a defence in English and Welsh law.
It means that the person who has committed the crime is
not in control of their own actions, and is usually used in
mental health-related circumstances.
At the same time it recognises that there is
some responsibility on the part of the person committing the crimes - if only that they did not take all reasonable precautions to prevent this or a similar situation from occurring.