Shamina Begum has never lived in Bangladesh and has apparently never done anything to avail herself of the rights and privileges of Bangladeshi citizenship.
Actions have consequences. She isn't showing remorse. Why should British citizens have to live with the fear that she might attack them or possibly pay for her time in jail (assuming that British jails are funded with taxpayer money) and/or surveillance?
Sucks for Bangladesh though. Their citizens are just as innocent in all of this.