Rachid El Omari’s awful comments were revealed when a Spanish judge sent him to prison after being quizzed in court.
He was one of a trio of Moroccan terrorists to be arrested after the plot emerged.
Ismael Moreno, in his committal order, said the probe which led to his arrest during a dawn raid “pointed towards him being in the process of planning a terrorist attack in Madrid.”
He added in the jail remand ruling, released yesterday, in which Rachid was accused of recruiting new members for terror group IS, also known as Daesh, it said: “His identification with the ideology of the terrorist organisation Daesh led him to justify the terrorist attacks in December in France.
“He claimed they got what they deserved and those who had pledged allegiance to the Caliphate like him had the job of defending Islam and converting everyone, as the Koran says those who don’t want to convert to Islam are infidels and have to die.
“He said the same things after the terrorist attack in Manchester on May 22 when it didn’t seem to worry him that children were killed because ‘it’s what they deserve and nobody complains about the children of Syria.’
“He went on to say, ‘Here something worse it's going to happen.’
“He also showed his interest in weapons, asking, ‘Would you know how to make a bomb?’”
Investigators have yet to disclose how they knew about the comments but it was understood they emerged through phone taps and surveillance of his internet conversations.
The sick comments were revealed as a reason behind the Spanish judge’s decision to remand El Omari, 32, in prison pending an ongoing investigation following his appearance at Madrid’s Audiencia Nacional court.
Two other suspects he stands accused of brainwashing and recruiting to IS include his 32-year-old cousin and a 37-year-old friend born in Tangiers, were also arrested with him at a flat near Madrid’s Atocha train station.
The judge said in his 14-page ruling outlining why he was keeping El Omari in custody that undercover police had seen him and another of the two suspects visiting symbolic places in Madrid on June 11.
He said the pair had kept watch on the undisclosed locations and resorted to “security measures” during their visits.
He added: “On June 14 they were seen in the early hours of the morning gathering up a poster with photos of women which the other suspect stamped on inside a doorway, while Rachid El Omari looked on.
“On June 13 Rachid El Omari left a note on paper with classic Arabic typeface in his workplace, after his contract ended.
“The note had a marked apocalyptic tone and could be translated as: “Jihad Dominion today returns to Paradise.”
Video footage uncovered by Spanish police revealed two of the three terror suspects being arrested in crash helmets outside their flat.
Officers said they had seized material including a manual for suicide bombers belonging to the main suspect.
In a statement, they said El Omari, who was not immediately named, “had an extremely dangerous profile, coinciding with those of the terrorists recently involved in attacks in the United Kingdom and France, and was considered an obvious threat to national security.”
Twenty-two adults and children were killed in the Manchester Arena terror attack, which saw suicide bomber Salman Abedi target fans as they left an Ariana Grande concert with a rucksack nail bomb.
A further 119 people were injured in the blast.
Eight-year-old Saffie Roussos was the youngest to tragically die that night.
Chris Upton, headteacher at Tarleton Community primary school near Preston, Lancashire where she was a pupil, described her as “simply a beautiful little girl in every aspect of the word”.
He added in a tribute after she was named as one of the victims: “She was loved by everyone and her warmth and kindness will be remembered fondly.”