Its an appalling tragedy. Ones heart goes out to all effected.
As an secondary aside already the clamour to understand why has started. "if we understand why we can prevent this".
This is an entirely understandable response. And its almost certainly doomed to failure. Those who spend time with the darker elements of human behaviour know that this "why" is just as unanswerable as " why" questions in the Buddhas day. There is no why necessarily, beyond pain and rage and despair and emotional numbness.
We can tinker with gun laws and speculate about kamma..but in the end its a mystery.
And thats scary.