She didn't go abroad to fight, IS recruits these girls to be sexual partners for its fighters, that's why the press calls them "brides". If a 15 year old girl had been groomed to run away from home to a location in the UK in order to have 3 children by an older man, without question that would be considered abuse and she would be regarded as a victim. Why should it make any difference that she went abroad?

She was also radicalised, both before and after she left the UK. To undo that sort of indoctrination/brainwashing takes expert intervention over a period of time. To expect a child (which she was until recently) to de-radicalise herself without assistance, and in an environment where the slightest indication of doubt in the cause or wavering of faith will bring down the most severe punishment, is completely unrealistic.

She should be allowed home. That is not to say she should not face the consequences of her actions, which may include jail, but that should also take account that she was a child who was taken advantage of by adults for their own purposes.