There's been a quote attributed to Mister Rogers that's been circulating on the Internet that says something like: whenever there's a tragedy, look for the helpers, see the good things that people are doing.
It's a basic law of physics that for every action, there is an equal and opposite reaction. Let's be that opposite reaction; let's be that force for good.
I'm reminded of the lines from "I Heard the Bells on Christmas Day" --
Then in despair I bowed my head:
"There is no peace on earth," I said,
"For hate is strong and mocks the song
Of peace on earth, good will to men."
Then pealed the bells more loud and deep:
"God is not dead nor doth he sleep;
The wrong shall fail, the right prevail,
With peace on earth, good will to men."