Following the assassination of Ismail Haniyeh in Tehran in July, Sinwar succeeded him as head of the political bureau, taking control of the entire group.

Israel-Hamas war latest

Believed to be the architect of the 7 October attacks, he was Israel's most wanted - a "dead man walking", according to Prime Minister Benjamin Netanyahu, who at one point claimed to have him "surrounded and isolated" in a bunker.

Just over a year since the most recent escalation in the region began, on 17 October the Israeli Defence Forces (IDF) confirmed it had killed the 61-year-old whose nicknames include "the face of evil", "butcher of Khan Younis", and "man of 12" - in reference to 12 suspected informers he was believed to have killed.

Granted fatwa by Hamas founder to kill collaborators

Sinwar was born in a refugee camp in Khan Younis, southern Gaza, in 1962.

He studied Arabic at the Islamic University of Gaza, which was founded in 1978 by the two men who went on to set up Hamas almost a decade later.

There he became particularly close to one of them, the cleric Sheikh Ahmed Yassin.

Yassin and Mahmoud al Zahar co-founded Hamas in 1987 as a Gaza-based political splinter group of the Muslim Brotherhood.

According to Israeli reports, Sinwar said Yassin granted him a fatwa (a ruling in Islamic law) to kill anyone suspected of collaborating with the Israelis.

He was first arrested for subversive activities in 1982. In prison, he met other key members of Hamas, including Salah Shehade, the former leader of its military wing the Qassam Brigades.

After being arrested and imprisoned again in 1985, he was put in charge of Hamas's internal security branch, the Majd Force, which sought out and killed suspected Israeli spies.

Dr Ahron Bregman, a former Israeli army major - and now senior teaching fellow in war studies and the Arab-Israeli conflict at King's College London, said: "The Israelis tried for many years to recruit him as a collaborator himself, offering him massive incentives.

"But it never worked with Sinwar. In fact he became notorious for killing Palestinians suspected of collaborating."

Learnt fluent Hebrew in prison

In 1988, he helped abduct and kill two Israeli Defence Force soldiers, which saw him sentenced to 22 years in an Israeli prison.

Despite being incarcerated, Sinwar used the time to his advantage - learning fluent Hebrew to better understand his enemy and ascending to become leader of Hamas prisoners in Israel.

Dr Bregman says: "He would read Israeli newspapers on a daily basis. He understood them way better than they understood him - hence his ability to deceive them and catch them off guard by executing his military operation so effectively in October 2023."

Fifteen years into his prison sentence, he went on Israeli television and spoke in Hebrew, calling for a truce with Hamas.

He was released in 2011 as part of the swap of more than 1,000 Palestinian prisoners for just one hostage Israeli soldier - Gilad Shalit.

Commenting on his imprisonment afterwards, Sinwar said: "They wanted the prison to be a grave for us. A mill to grind our will, determination and bodies.

"But thank God, with our belief in our cause we turned the prison into sanctuaries of worship and academies for study."

Forced suspected informer to bury his own brother

Back in Gaza he continued to increase his influence among Hamas's highest ranks.

He remained committed to his original task of unmasking and killing traitors - both Israeli collaborators and members of rival militant groups.

A former member of Israeli intelligence told the Financial Times that he once boasted about forcing a Hamas member suspected of informing for a competing faction to "bury his own brother alive… handing him a spoon to finish the job".

In 2015, he is thought to have been involved in the torture and killing of fellow Hamas commander Mahmoud Ishtiwi.

He was accused of embezzlement and "moral crimes", including alleged homosexual activity, with Sinwar thought to have orchestrated his murder over fears he could compromise the group.

Commenting on how he killed another collaborator, he told how he and a group of others blindfolded Ishtiwi and drove him to a makeshift grave, before strangling him with a kaffiyeh (Arabic male headdress) and burying him there.

Read more:
Poll reveals what Britons think of Armistice Day protest
Satellite imagery reveals how Israel has cut Gaza in two

'Mythical figure' in Palestinian history

The same year he is thought to have killed Ishtiwi, he was designated a terrorist by the US government.

He replaced Haniyeh as Hamas leader in Gaza in early 2017 and was re-elected in 2021, later surviving an assassination attempt.

Three years later, following Haniyeh's own eventual assassination in Tehran, Sinwar would succeed him once more as political chief.

As leader he increased the group's use of force, stepping up protests and rocket fire at the Israeli border.

With his military background, he was seen as someone capable of uniting Hamas's armed and political wings.

Dr Bregman described him as a "man of few words" and a "natural leader… charismatic, secretive and manipulative".

"He will be remembered as the architect of the 7 October attacks and the person who inflicted on the Israelis their most terrible disaster since the establishment of their state in 1948," he added.

Israel's chief military spokesman, Rear Admiral Daniel Hagari, also blamed Sinwar for the 7 October attack.

Although his methods were "barbaric", Dr Bregman believes 7 October will be seen, "from a Palestinian point of view, in spite of the terrible price they are paying now, as a great victory".

"Sinwar has earned a place in the pantheon of great Palestinian leaders."

Testimonies from people on the ground in Gaza, however, suggest his violent methods have left many of them disillusioned with Hamas.

With Israel's promise to destroy Hamas and all of its leaders, Dr Bregman said it was inevitable it would "get him in the end" but that "there is no doubt Sinwar will go down in Palestinian history as a mythical figure".