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An ‘Irish jihadi’ fighting with ISIS has justified the execution of David Haines in disgusting online posts.

The fanatical convert, who describes himself as “half Irish-half Nigerian", claimed the British aid worker’s former job in the RAF meant he was a target.

The extremist, who has turned his back on his Irish roots, said Haines could have been saved if the British Government had negotiated.

The jihadist, using the name Muthenna ibn Abu, was writing on the social media website Ask.fm. His account has since been suspended.

Muthenna, who is believed to be fighting for ISIS in Syria, wrote: “David Haines was in the RAF for 10 years, the same RAF that bombs Muslims and gets involved in wars in the Muslim lands that has no direct threat and has nothing do with Britain.

“Have you seen the track record of USA and UK how many innocent Muslims they have imprisoned and killed?

“Not to mention the occupation and killing campaign they support in Palestine.

“I love how the amount of westerners killed can be counted on one hand but the blood of the Muslims can run like and open tap and no one cries for them.”

One furious Ask.fm user asked him: “What did British aid worker David Haines do to your pathetic murdering army?”

He replied: “Murdering army, lol (laugh out lout). Don’t make me laugh.

“At least we give your government the choice to save your fellow countrymen, I couldn’t say the same for your ‘murdering army’.

“Your government is hypocritical and they are brainwashing your public down the same road.

“They actually don’t care about you, the family of (James) Foley was warned that if they raised money for his realise (sic) then they could be prosecuted.

“Many hostages from other western countries was freed because their governments took the smart choice instead of being arrogant.”

Muthenna, who describes himself as “half-Irish half-Nigerian” has previously described his life under ISIS – posing with AK-47 assault rifles.

He has encouraged Muslims as young as 16 to travel to Syria and Iraq to join the fanatical group.

He has been posting messages from Syria since early July and also uses an encrypted texting service to try to persuade others to join the jihad.

Muthenna says he is a Muslim convert and has described his own parents as “kuffars” (infidels).

A video showing Mr Haines being executed was released on Saturday night.

Mr Haines, a 44-year-old father of two, was in the RAF for 12 years before becoming an aid worker, travelling to some of the world’s most dangerous areas, including Libya, Sudan and the former Yugoslavia.

He was abducted in March 2013 just three days after arriving in Syria, where he was looking for sites for possible refugee camps for a French charity, and was handed over to ISIS.