The 19-year-old, who is pleading to be repatriated from a detention camp in Syria, told the BBC it was “wrong that innocent people did get killed” in the 2017 atrocity.
But she compared the Manchester bombing to the “women and children in [Isis territory] being killed right now unjustly with the bombings”, adding: “It’s a two-way thing really because women and children are being killed in the Islamic State right now and it’s kind of retaliation. Their justification was that it’s retaliation so I thought OK that is a fair justification.”
The group’s propaganda used civilian casualties in airstrikes and alleged war crimes by its enemies to attract recruits and justify its own atrocities.
Ms Begum told the BBC she joined Isis aged just 15 in 2015 after watching its propaganda: “Not just the beheading videos, the videos they show of families in the park you know, the good life.”
She acknowledged her story may have inspired others to join Isis, adding: “I did hear a lot of people were encouraged to leave after I left but I wasn’t the one that put myself on the news.”
While Isis became notorious in the west for its footage of gory executions, including the murder of British and American hostages in 2014, much of its propaganda claimed to show a utopia awaiting recruits in the caliphate.
Ms Begum married a Dutch foreign fighter called Yago Riedijk, who is currently in the custody Syrian Democratic Forces (SDF), and had two children who died as infants.
She gave birth to their third child on Saturday and is pleading to return to the UK.
It remains unclear whether any legal move could stop the British citizen re-entering the UK, but Syrian authorities say she will not be allowed to leave the al-Hawl camp without official assistance, and the Foreign Office has refused to give it.
Shamima Begum on moving to Syria: 'Videos on the internet attracted me to join them'
Theresa May rejected a call from Donald Trump for the UK and other European countries to take back their citizens, or risk the SDF letting them go.
Ms May’s official spokesperson said: “Foreign fighters should be brought to justice in accordance with due legal process in the most appropriate jurisdiction.
“Where possible, this should be in the region where the crimes had been committed.
“We continue to work closely with our international partners on this. The government will do all it can to ensure the safety and security of the UK.”
Law enforcement officials have said Ms Begum and any other returnees from Syria will be investigated and charged if a prosecution if possible, while police are also continuing attempts to stop extremists travelling abroad.
Ms Begum’s family lawyer, Tasnime Akunjee, anticipated Ms Begum would face criminal proceedings upon any return to the UK, but said it was the family’s hope that she would be given professional help.
In a press release issued on Monday, Mr Akunjee blamed a “litany of failures” by Tower Hamlets Council, Ms Begum’s school and the police for letting her travel to Syria.
Ms Begum was one of three schoolgirls, along with Kadiza Sultana and Amira Abase, from Bethnal Green Academy who left the UK for Syria in February 2015.
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