If she comes back and is free then a lifetime of surveillance will cost a fortune, but if she is popped off in a revenge attack, the surveillance costs stop, but then the cost of imprisonment of the hitman/hitwoman/hitperson will cost a fortune.
Denying her her citizenship is probably the cheapest option in the long run, from a financial perspective.
From a moral perspective, she was born here so we should deal with our own shit, but as her parents are from Bangladesh (iirc), she would have access to Bangaldeshi citizenship officially if they could be bothered to accede.
As a general aside, I see lots of mention in posts above of nothing being done to stop the grooming like this (if she was groomed). Do people not realise that the security services are overrun with dealing with, stopping and monitoring grooming, terrorist, nutjob shit all over the huge worldywideyweb internet, The security services stop plenty, but they can't catch everyone!