I don't think anyone is making the argument that there should be no justice. Prosecuting the criminals who commit murder for the purpose of terrorism is clearly a "no reasonable person could disagree" sort of a thing.
But the question remains, how is justice won? In theory we could impose martial law and suspend elections, or permanently shut all the roads and trains and prohibit gatherings of more than five people in the same place, or nuke the entire middle east. Perhaps doing those things could bring justice to more terrorists.
Well before we reach that point, we come to a line we should not cross. We come to a choice that will cost us more of our humanity than it gains us in justice. And whipping the public into a frenzy is how popular support for crossing the line we should not cross is manufactured.
That's why people care about "justice" for murderers even though more people are killed by auto accidents. Murder upsets the social order. A car accident doesn't.