They have a responsibility to ensure their citizens aren't slaughtering people in Syria.

Not being funny but if I decided to go and kill people in another country - say the US - wouldn't I become the US's problem and be convicted under their laws etc?

Reference the the British person in Dubai in the last couple of days who is going to prison - in Dubai - for brushing against another man. Why isn't he Britain's problem?

These people are acting in their own name, not in the name of Britain after all.

As for prison, if there's any evidence that they committed crimes abroad, they should be tried in accordance with the law. Otherwise, the best you can do is monitor them.

If you're coming back from Syria right now I wouldn't have much of an issue with there being some very strict protocol around how long you can be held while such evidence is gathered.

Such things - in this extreme instance - seem worth the risk to stop potential extremist militants - even murderers - coming back and walking the streets like nothing happened.