I didn't say that the actions of the British government are responsible for them joining a rape and genocide cult, the actions of the British government in invading Iraq created the conditions that allowed the rape and genocide cult they went on to join, to commit mass rape and genocide.
The possible outcomes are either they are killed, imprisoned or let go. All three possibilities will create a burden on whatever region they occur in, and by revoking their passports the British government has shirked their responsibility and placed that burden on someone else. In Begum's case that is either an extra burden on the area of Syria she is now residing, or on Bangladesh, both of which are much more ill-equipped to deal with her then the UK. In the case of Lett's, if the Kurdish forces want to relieve themselves of the burden by handing him over, then there is no justification fir the UK refusing.