In an interview with the BBC, Shamima Begum, now 19, said the deaths of 22 innocent people in the terrorist attack on an Ariana Grande concert in 2017 were akin to the "women and children" being bombed in IS territory in Baghuz.
She told the broadcaster: "I do feel that it's wrong that innocent people did get killed. It's one thing to kill a soldier that is fighting you, it's self-defence, but to kill the people like women and children...
"Just people like the women and children in Baghuz that are being killed right now unjustly, the bombings. It's a two-way thing really.
"Because women and children are being killed back in the Islamic State right now and it's kind of retaliation. Like, their justification was that it was retaliation so I thought 'OK, that is a fair justification'."
The mother-of-three, who gave birth to her third child at the weekend, left east London with two friends in 2015 to join the terrorist group.
She was partly inspired by videos of fighters beheading hostages and partly by other propaganda films showing the "good life" IS could offer.
Since she has been there, her two older children have died.
The teenager insisted she did not ask to be the subject of international media attention.
She said: "I didn't want to be on the news at first. I know a lot of people, after they saw that me and my friends came, it actually encouraged them.
"I did hear, yeah, a lot of people were encouraged to come after I left but I wasn't the one that put myself on the news. We didn't want to be on the news."
She later told ITV News she did not see why Home Secretary Sajid Javid would see her as a threat.
"I'm a 19-year-old girl with a new born baby. I don't have any weapons; I don't want to hurt anyone even if I did have weapons or anything," she said.
"He has no proof that I was a threat other than that I was in ISIS, that's it.
"I don't know how I would be seen as a danger. I'm not going to go back and promote people to go to ISIS or anything, if anything I'm going to encourage them not to go because it's not all it seems in their videos.
"I do regret it because when I went I thought I was going to make a family and I didn't realise what the things they were doing that they weren't showing in their propaganda videos, and I actually do regret it, I do feel bad for anyone who was affected by the actions of ISIS."
UK authorities now face the difficult question of what to do if Ms Begum manages to return to Britain.
Her family's lawyer, Tasnime Akunjee, said he anticipated she would face criminal proceedings upon any return to the UK, but that it was the family's hope she would be given professional help following her experience in Syria.
He told BBC Breakfast: "The family have gone out of their way from day one to try to get her away from the IS narrative and the context which she finds herself in.
"She's been there for four years and we would be surprised if she hadn't been further damaged beyond the degree she had already been groomed into.
"The family are concerned, as they have been for the last four years, not just to get her away, but, as of yesterday, to make sure that their grandchild - her child - is not influenced by that sort of thinking."