By Sara Elizabeth Williams and Abo Bakr Haj Ali

A key West-backed Free Syrian Army (FSA) commander was assassinated late Thursday in a small village in south-western Dera'a. Captain Qais al-Qatanah, the leader of the al-Omari Brigade, was shot twice in the head and once in the leg by a self-styled media activist named Kaesar Zezoon after an altercation, according to sources close to the commander.

Al-Qatanah was rushed to the border and then on to Jordan’s Ramtha Government Hospital, but was pronounced dead on arrival. He was buried Friday in the village of Baj in Mafraq, Jordan.

A message posted on the al-Omari Brigade’s Facebook page says Zezoon approached al-Qatanah, saying he wanted a word with the man. According to the message, al-Qatanah told Zezoon to come by the group’s headquarters to talk, saying “My office is open to you and to others, 24 hours a day.”

The message alleges that Zezoon rejected this offer and opened fire. Zezoon was then shot by al-Qatanah’s security detail and taken to a Dera’a field hospital where an insider said he is under guard with non-life-threatening injuries. On Friday, he posted a hospital bed confession to YouTube.

In the hours after Zezoon’s confession, the al-Omari Brigade posted a statement to its Facebook page insisting that they will wait for the local sharia court to pass judgement before they take any actions. The brigade is unmistakeably urging caution until it is clear who Zezoon was acting for or with.

For those watching southern Syria, the last FSA stronghold and a place where tensions between rebel groups are high and rising, there is so much déjà vu.

“Qatanah's death comes amid several months of reports and rumours in the south that Jabhat al-Nusra has initiated a covert campaign aimed at undermining these largely Western-backed moderate groups,” said Charles Lister of the Brookings Doha Center.

“There have been several similar incidents of commander deaths in Dera'a and also in Amman in recent months and many group leaders are said to have either moved permanently to Jordan or to have adopted tight security procedures for their own safety.”

Al-Qatanah was one of those leaders. Sources close to him said he slept in a different place every night and had done so for at least the past year. He began each day by checking the underside of his car for sticky-bombs.

As al-Qatanah’s influence grew to the point that he commanded 2,000 men from the border to the southern fringes of Damascus, so did his fears that he might be assassinated. Close friends say he worried constantly about his own safety.

Yet he continued to operate in Syria, despite the risks. Southern Syria was al-Qatanah’s home turf and the place where his influence was strongest. As a Bedouin, he was able to reach out to that tribal network and muster critical support.

Abu Waleed, a press officer with the al-Omari Brigade, said al-Qatanah’s death will leave a “huge vacuum” in the region.

“In regards to fighting against the regime of Bashar al-Assad in particular, he was a safety valve across most of the region and was able to consolidate good relations with Horanis and with our neighbours, the Druze community in Suwaydah," Abu Waleed said.

He also brought a sense of military legitimacy to rebel ranks: al-Qatanah was one of the first Syrian officers who defected from the Syrian Arab Army at the start of the conflict, and the brigade he formed was one of the first armed opposition groups in Dera’a.

The al-Omari Brigade is an armed opposition group with secular leadership and a secular, democratic vision for Syria. The group is not associated with al-Qaeda, like Jahbat al-Nusra, nor with the Islamic State. Although the al-Omari Brigade and other FSA chapters have fought alongside al-Nusra in the past, their methodology in the field is markedly different. Under this vein of FSA leadership, there is an adherence to sharia law but there are no public executions, declarations of religious war, state-mandated religious adherence or orders for women to remain indoors.

Today the bridgade is one of the last such rebel groups with any real power in the south, and as such, a crucial touchpoint for Western interests along Syria’s southern front. Over the past year or more, the brigade benefited from American arms - including TOW missiles - and military expertise. Scores of al-Omari fighters are reported to have attended the covert American-run training camp in Jordan, some as recently as in May of this year.

But with the rise of the Islamic State, tensions between those who welcome Western support and those who want to eliminate it completely have climbed higher than ever. This assassination, Lister believes, is likely to be part of the “existing hostile dynamics between senior leaderships in the south, particularly between Western-backed moderates and jihadists.”

The element that has yet to emerge fully in the showdown that looks more and more likely to happen in Dera’a is the IS. Sources across Dera’a speak of sleeper cells slowly reaching out to the more worn-down fighting factions, offering money and support to fall in with Da’esh as the Islamic State is called in Arabic.

“There are growing rumours that some elements within Nusra bear some loyalty to the Islamic State and may be preparing the conditions for the group's emergence in the south,” said Lister.

As the conflict between rebel groups in Syria's south becomes ever bloodier, it begins to look possible that al-Qatanah's assassination may be one of those conditions.