The defence of provocation, which can be used to reduce murder to manslaughter, has been abolished.
Parliament passed the repeal bill last night by 116 votes to five, with only the Act Party opposing it.
Justice Minister Simon Power introduced the Crimes (Provocation Repeal) Amendment Bill in August after the Law Commission had twice recommended abolition.
He said at the time the Government considered the law was flawed.
"It effectively provides a defence for lashing out in anger, not just any anger but violent, homicidal rage," Mr Power said.
"It rewards lack of self-control by enabling an intentional killing to be categorised as something other than murder."
The provocation defence came under debate after Otago University tutor Clayton Weatherston claimed he was provoked into stabbing ex-girlfriend Sophie Elliott 216 times.
Weatherston denied murdering her, but admitted manslaughter. The jury found him guilty of murder.
Labour, the Greens and the Maori Party supported the repeal bill and there were cries of "shame" when Act MP David Garrett said his party opposed it.
Mr Garrett said the central point in the Weatherston case was that the defence failed.
"Abolishing it suggests we don't trust juries," he said.
The repeal shifted the argument of provocation from juries to judges, who can consider it in sentencing.
"It makes more sense to put our trust in juries," he said."