Changes to the law of murder have come into force in NI, allowing domestic violence victims to claim they were forced to kill their abuser.
They can avoid a murder conviction if they can prove they were motivated by "words and conduct" which left them "seriously wronged".
It replaces the defence of provocation with one of "loss of control" caused by a fear of serious violence.
Justice Minister David Ford said the changes made the law more fair.
The changes, under the Coroners and Justice Act 2009, bring the law in Northern Ireland into line with England and Wales.
Men and women who kill after long-term abuse could use one of the partial defences, which replaces a requirement for them to have acted on the spur of the moment.
Under the changes, the partial murder defence of diminished responsibility has been replaced with "a modernised definition", based on the concept of "an abnormality of mental functioning which arose from a recognised medical condition".
Previously, adultery could count as provocation - a "crime of passion" - but under the new loss of control defence, sexual infidelity - "real or imagined" - on the part of the victim does not constitute sufficient grounds for reducing murder to manslaughter.
Mr Ford said the changes were intended to help deliver fairer treatment for victims of domestic violence.
"The unlawful taking of the life of another human being is the most serious issue which our criminal courts have to deal with," he said.
"These changes update the law, including where murder is linked to domestic abuse, and will help to provide a more just and equitable outcome in individual cases."
Last December, the lord chief justice of England and Wales criticised the changes after they came into effect there.
Lord Judge said scrapping provocation as a defence in murder trials and replacing it with the defence of loss of self control was not "sensible".
He told the Lords Constitution committee it was overly complex and suggested it could cause confusion.