Magus on a Star Trek wrote:
The rule was, ďAt the first sign of interference from the outside world, [the trigger man] will trigger the bomb.Ē At the time, Bruce was in the outside world. Why didnít Talia push the trigger as soon as he returned?
Because the whole point was for Bruce to watch Gotham be destroyed, to see his failure. It doesn't torture his soul if just randomly explodes.