Mental illness should be a part of the trial and sentancing process. People who have mental illness that cause them to commit crimes or who do not understand the consequences of their actions should be treated differntly than people who are functioning at a normal, fully capable level like everyone else.
The man in question is a suffering paranoid schizophrenic. This disease is debilitating to the mind. A person would not know what they were doing half of the time. The man should not be executed. He needs help. As it is here in the United States, mental illness should be a factor in determining crime sentencing everywhere.
Part of the process of a serious crime should be that the person intended the harm and that they are a threat in the future. A mentally ill patient in normal circumstances did not fully understand the action at hand and either could be put somewhere they can be safe and not harm others or could be taught not to do something. If the mentally ill patient cannot be put anywhere that they would not harm themselves or others and their is no way to control them, then they could be executed.
No, I don't believe that mental illness should be a factor in determining sentencing. There are unfortunately times when someones mental illness will cause them to commit dangerous crimes. Although their mental illness may be to blame, that doesn't make them any less of a threat to society. In fact, their mental illness could make them more of a threat for continuous violence in the future.