Not a bad conscience as such. I suspect they do have a conscience, but it is guided by a bloated sense of Truth born from ignorance, resentment and, perhaps most of all, the desire not to think, not to reason — a deeply human desire, I'm afraid. This Truth justifies all their actions. But I also suspect that somewhere deep, deep down they know very well that what they believe is nonsense, founded on a fantasy world. A glimmer, indeed, outshone by classic delusion. Like the cliché of the psychiatric patient who is convinced he is Napoleon. Contradicting such a person tends to make them very angry.
The terrorists prove nothing. Perhaps they think they are at peace the moment they explode themselves in the name of their religion and then hopefully they face an eternity of torture.
I find it impossible to imagine what goes on in a terrorist's head and almost dangerous to try. What pains me is to see the shrill and shallow headlines calling the horror "the deadliest attack in France since WW2". It made me think, almost callously, what might have been the next deadliest. Could it have been the killing by Paris police in 1961 of at least 100 Algerian demonstrators?