Killing when provoked is "truly understandable and perhaps human" in some circumstances, ACT leader Rodney Hide says.
ACT last night failed to vote down the Crimes (Provocation Repeal) Amendment Bill.
The bill, which scrapped the partial defence of provocation for murder, passed 116 votes to five.
Hide told Parliament that the law change was prompted by the Clayton Weatherston case. Weatherston, 33, argued, unsuccessfully, that he was provoked to kill Sophie Elliott.
"The country was rightly horrified by this vicious and brutal attack on a young woman," Hide said.
"We were mortified that Clayton Weatherston would attempt the defence of provocation. But from that does it follow that we should get rid of that defence?"
Hide said people should consider what they would do if they had discovered Weatherston standing over Elliott.
"What would members of this Parliament do? Well, we don't know actually what we would do.
"I can't imagine what that would do to your mind to see your sister or your daughter or a loved one at that moment so brutally butchered on the floor.
"It seems to me that a reasonable man might, in a fit of rage, kill the bastard. And then you're on a murder charge.
"But haven't you got a defence that you were provoked by the actions of that terrible man?"
Labour MP Lianne Dalziel said she was sorry the bill did not pass unanimously.
She said it was important that Parliament could say to victims' families who had experienced a provocation defence that their experiences had "galvanised a response".
Dalziel said the use of provocation in the Ferdinand Ambach case was "reprehensible in the extreme".
Ambach, 31, was this year convicted of manslaughter after successfully using the partial defence of provocation in the killing of Ronald Brown, 69.
It was alleged Brown made homosexual advances towards Ambach, who reacted by bashing him to death.
"There is nothing that is going to reverse what has happened in any of the cases where this defence has been pleaded," Dalziel said.
Evidence in the Clayton Weatherston case has been sent to Wellington lawyer Robert Lithgow, QC, to prepare an appeal against his sentence.
In September, Weatherston was sentenced to 18 years minimum non-parole for Sophie Elliott's murder.
Lithgow yesterday said Weatherston's appeal would not proceed until well into next year.
"All of this takes time.
"I'm sure it's not easy for the [Elliott] family, who want it all done and dusted, but it's not going to be like that, I'm afraid."
- The Press
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