What I would say is that God tends to let things happen unless absolutely necessary, and we don’t know what “absolutely necessary” is because we aren’t God. Hurricanes happen because the prerequisite for “no hurricanes” would be considerably less water in the oceans, considerably less heat, or a mass rewrite of the laws of physics, all of which are bad. Earthquakes and such happen for a similar reason.
Human disasters like 9/11 happen for a similar reason. As we stand, humans have amazing, wonderful tools at our disposal, and we can build grand and wonderful things. But humans are also fallen, and as such, we can also corrupt and destroy, and sometimes other people are in the way.
I follow Liebniz’s “best of all possible worlds” model of thought on this one – sure, this may suck, but any option which would have prevented it would be
much worse – either we’d live in a completely incoherent universe, be mindless love robots (which is a contradiction in terms), or simply wouldn’t exist.
It’s not much of a consolation, but it’s what I have.
P.S. What’s freaky is that I’m old enough to have a rational discussion with y’all about this, but I don’t remember 9/11 at all.