Live and learn
According to this mornings The Wright Stuff on five, a spouse cannot give evidence against their partner in a court of law, unless the partner have committed a crime against the spouse, or against a person under the age of 16.
So, if a wife was the only witness to her husband committing a murder, she could not testify against him. Although I can (to some extent) see why the law works this way (how can a spouse be relied upon to give dispassionate testimony against a loved one in every case?), I can’t imagine what it must be like to be married to someone who commits a truly evil act, and not be allowed to testify.