But surely if perpetrators are captured, they must be punished? (And I don't mean the Death Penalty) What's the alternative? Sitting them down over a nice up of tea and having a little chat about why they're so cross with us?

On a wider front, the whole situation does need looking at in depth. Alienated groups can become dangerous (as seen) and it is a good idea to unpick the chain of events which led to this. But from what I've read about IS, they aren't in any way prepared to consider reconciliation or talks. And individuals who plot atrocities, and those with bomb-factories in their houses MUST be brought to justice and punished.