God's gift to the human race was free will. He couldn't intervene because it would interfere with a humans right to chose what they wanted to do
Pretend for a minute that God is a gun shop owner who can see the future.
A man walks into the gun shop and asks for a gun. However, the gun shop owner knows, through his ability to see the future, that this man will use the gun to murder some other person. Nevertheless, he sells him the gun, stating in his defence that he doesn't want to infringe upon the man's free will.
Do you think in this scenario the gun shop owner should be absolved of any responsibility?
The only prima facie plausible justification for the existence of evil I've heard is the idea of free will, which is actually absurdly easy to pick holes in.
Could we not assume that 'God', being omniscient, could see the benevolent direct/indirect effects of 9/11 (that humans can not/haven't yet foresee) that could out weigh the non benevolent effects in the short term (mass loss of life)