Is taking hostages, especially of non-combatants such as Kenji Goto, permitted in Islam? I'm especially interested in the Sunni or Salafi viewpoint.
Here's an response from a historical perspective.
For starters, it's not considered academically judicious to explain religious concepts using contemporary political movements. Go to the scriptural texts for your authentic answer. The older it is, the closer you will be to the source.
Having said that, during the first battle of Islam - known as the Battle of Badr which the Muslims won - prisoners of war from the pagan Arabs were taken by the Muslims.
These captives, many of whom had persecuted the Muslims for leaving their religion by the use of torture, abduction and murder, were instead treated in a humane and compassionate manner - much to their own surprise.
It is documented that the Muslims fed the pagan captives with baked bread - a luxury in those times - and sustained themselves with dates, which was the staple food.
However a condition was stipulated on the prisoners of war, many of whom were wealthy and therefore educated. "Teach ten Muslims to read and write, and you will be set free." A number of the pagan captives jumped at the opportunity and consequently the Muslims kept their word and emancipated them.
Using the above narrative, it MAY BE inferred that hostages are allowed in Shari'ah, but they were not tortured or beheaded (even though they had tortured and killed many Muslims beforehand), but were instead afforded the kindest treatment and the best food available at the time.
The above conditions of compassion and mercy have to be upheld even in this day and age by the Muslims if they choose to retain hostages or prisoners of war.