Mahmoud says he found a note belonging to his sister with several phone numbers and rang one to find it was a clinic that restores a woman's virginity through surgery.
He says that was the final straw.
"She was violating rules of the society. Why has she done that?
"She didn't have the right to do that. She shamed our family."
On average in the West Bank, about 20 men a year like Mahmoud kill a sister, daughter or even their mother because they claim she has brought shame on the family.
Some women are killed if they are considered promiscuous, have an affair, or even if they are victims of rape or incest.
"I don't think if anyone betrays his family (they) can be trusted by anyone else," Mahmoud said.
He says his views are representative of most people in the West Bank.
Mahmoud spent six days in custody. He was never charged and does not see himself as a murderer.
"It wasn't my fault, and I haven't done anything wrong," he said.
"I overreacted but I don't think that it makes me deserve jail for a long time."
'Killing is killing'
But human rights groups, non-government organisations and virtually every minister in the Palestinian government is demanding change so others like Mahmoud are treated as cold-blooded murderers.
Ashraf Hayyeh from the Palestinian Women's Centre for Legal Aid is one of the strongest advocates for reform.
"Killing is killing, whether it's for honour or anything," she said.
"And if someone kills that way they should deal with it as murder. He should be sentenced to at least 15 years' jail."
There is, however, one major obstacle to change.
The Palestinian penal code, in the West Bank at least, is a Jordanian law drafted 50 years ago.
To change it the Palestinian government needs to abolish that law and draft its own.
But since the militant group Hamas refused to take part in elections due last January, the Palestinian Legislative Assembly is effectively paralysed.
Until there is a new election, men like Mahmoud will continue to be let off the hook for what most countries would say is cold-blooded murder.
BEASTS WHO BELIEVE IT'S OKAY TO MURDER THEIR OWN DAUGHTER AND SISTERS TO RESTORE FAMILY HONOR CANNOT BE TRUSTED WITH ANY PEACE AGREEMENT AND SHOULD NOT BE GIVEN YET ANOTHER STATE.
THEY CANNOT BE AND MUSTN'T BE TRUSTED. SINCE THEY ROUTINELY MURDER THEIR OWN ARAB/MUSLIM SISTERS, THEY OF COURSE CAN SHOOT PREGNANT ISRAELI JEWS FOR NO OTHER REASON THAN THE FACT THAT THEY ARE JEWISH ISRAELIS.
THEY SHOULDN'T BE GIVEN A STATE. THEY DON'T DESERVE ONE. IT WOULD BE A TERROR STATE.
THAT'S JUST THE WAY IT IS. THAT'S JUST THE WAY IT REALLY IS.