She was born in Britain and had a British passport by birth. She wasn't awarded citizenship as an immigrant because she didn't immigrate here. She is British, and has never held citizenship of any country other than Britain and therefore I don't think Britain should be able to remove the citizenship of anyone who has an automatic right to it by birth, under any circumstances.
She has committed a terrible crime and I think she should pay the same price any other British criminal would pay. When someone like, eg, Rose West or Levi Bellfield commits a crime, we deal with them with a criminal trial and sentence. We don't just announce that they're no longer British and refuse them a trial. Therefore we shouldn't be doing that to Shamima Begum either.
Whether or not she was groomed or vulnerable at the time of her crime and whether her age was relevant should be neither here nor there in terms of her citizenship. Even if she knew exactly what appalling crime she was committing and wasn't manipulated or groomed at all, she still shouldn't have her citizenship by birth taken away on that basis. Whether she was groomed or not should be an argument put forward in a criminal court (as it would be for any other defendant who committed their crime when they were 15) and the judge can consider whether it is or isn't a mitigating factor when sentencing.
I've got virtually no sympathy with her on a personal level, in fact. I don't think there are any excuses for her crimes. But I do think it's a slippery fucking slope if we just start telling British people they're not British in lieu of giving them a trial. I think a lot of people who think the withdrawal of citizenship is OK only think it's OK because she's non-white, and haven't realised that that this could happen to any British-born person, not just British-born people with dark skin and immigrant parents.