Shamima Begum should absolutely be allowed to return to the UK.
It is a violation of international law to strip someone of their citizenship if doing so renders them stateless. The UK claims Begum is Bangladeshi, but this is not true. She does not have Bangladeshi citizenship and she is not welcome in Bangladesh.
What is the legal basis for stripping Begum of her citizenship?
Begum was recently featured in a documentary, The Return: Life After ISIS, along with several other women from Western countries who joined ISIS and are currently being held at the same refugee camp in Kurdish Syria. She describes how she was brainwashed into joining ISIS and how she was lied to about what it was going to be like. She says that in previous interviews she felt pressured into saying certain things because she was afraid of other people in the camp.
From a humanitarian perspective, Begum has suffered tremendously. She joined ISIS at 14 or 15 years old and was almost immediately forced to marry. She had 3 children while still a teenager and in the chaos of war all 3 of them have passed away from illness and a lack of food and clean water and medical care.
As a teenage girl and a newly practicing Muslim who doesn't even speak Arabic, there is no reason to think Begum participated in any terrorist acts.
Whatever criminal charges there may be, if any, Begum should face them in Britain.