Now known as Muthenna ibn Abu, the fanatical Muslim convert claimed on website www.Ask.fm that Haines was a legitimate target.
Muthenna, believed to be in his 20s, is currently fighting with ISIS in Syria and has denounced his parents, one Irish and one Nigerian, as infidels.
He has also encouraged Muslim teenagers as young as 16 to join him and fight in Syria.
He wrote on the website that aid worker Haines’ former role in the RAF justified his brutal beheading.
He 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 to 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.”
The website ask.fm has since suspended the fanatic’s account.
Muthenna was asked by one poster to the site: “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.”
Describing himself as ‘half-Irish half-Nigerian’, he has previously described his life under ISIS on the site and posed with AK-47 assault rifles.The Irish Independent reports that Muthenna says he is a Muslim convert and has described his own parents as ‘kuffars’ (infidels).
Father-of-two Haines 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.
He was handed over to ISIS who released a video of his beheading at the weekend.