Lest this 'hyper-Hegelian' view seem utterly moronic, it is plausible (though not certain) that, at least at the macroscopic level (to avoid problems with quantum physics), all things are caused. This doesn't mean that nothing is mysterious, but it does mean that mystery can come in two major varieties: stuff we don't know, but about which there was a fact of the matter (for example, the number of hairs on Abraham Lincoln's head at the moment he was shot is a mystery, but only in this relatively mundane sense); and stuff which involves more than merely understanding the physical causes. For example, even knowing exactly what physical cause produced the avalanche that killed your dog, you might still wonder why things had to be so arranged that your dog died. My general feeling is that mysteries of this second type are actually meaningless--it is possible, for example, for there to be no plan, and there would then be no meaningful answer to give other than an account of the physical goings-on. That's a guess, though.

In any case, there seems to be no reason that the assumption that all things are caused is wrong, and it seems to be a fairly optimistic, humanistic belief. Rather than presuming that there are things we could not possibly understand (a seemingly defeatist attitude), it assumes that nothing is beyond us, in principle. It also allows us to DO something about our problems--if we can know why school shootings happen, we can prevent those circumstances from coming about.

There's an additional question, with this particular issue, of why so little attention is paid to those who make the shooter feel ostracized and tormented. I don't know the answer to that--Hegel seems to be failing us. :-)